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Discussion Starter #1 (Edited)
This is a topic that I've been thinking about for some time. Have some design options in the works but thought I'd share with all of you smart MOF friends to start what I hope will be some informative and interesting dialog.

Performance Firearms and Thermodynamics. Part I
Unless you work as an engineer, you probably haven’t given much thought to thermodynamics since your last physics class. If you bring up the topic whilst hot tubbing, people will start putting their clothes back on. It is a fascinating subject, but perhaps a little dry for hot tubbing. In industries focused on performance machines, thermodynamics is a vital science. If you’re in it to win it, ignorance of the subject will result in your complete annihilation.


Having spent 15 years in the high performance motorcycle industry, I am now puzzled by the reality that the firearms industry has neglected this science for so long. After all, two truths are, or should be, understood by now: 1) firearms generate a lot of heat; and 2) heat is a barrel’s worst enemy. So why has so little attention been given to disposing of that heat? To make matters worse, the industry itself often misinforms consumers and muddies the waters. But I’ll get back to that. For now, let me illuminate some basic principles, drawing on my background in high performance motorcycles. Guns are essentially single stroke engines, so there are plenty of analogies.

Compared to the firearms industry, motorcycles evolve at a breathtaking pace. It wasn’t always that way, but these days there is an enormous amount of money at stake. Honda, far and away the global leader, sells roughly 15 million units annually. They and their competitors understand that success on the race track sells bikes. So these are very serious battles, not just by the riders, but by the hundreds of engineers and support staff behind the riders. The most serious arena of all is Moto GP, the premier class, equivalent to F1 in auto racing. Factories will spend hundreds of millions to compete, and find some technological edge, at this level. And not just motorcycle factories either.

Tire companies like Bridgestone and Pirelli will also spend millions in development to ensure that their tires are on the winning bikes.Tire engineers study thermodynamics a lot. They are the unsung heroes of the sport. The best engine, chassis, and rider will get nowhere near the podium without a reliable way to transmit all that power to the tarmac. And all that power transmission to the tarmac generates—you guessed it—lots of heat. Tires have an optimal operating temperature. Too cold and the rubber compound doesn’t deform well enough to grip the road surface. Too hot and the compound can become “greasy” and unpredictable, or deteriorate too rapidly to keep the rider in the race. Tire warming jackets are used in the pits to ensure that the optimal temperature is achieved before the bike hits the track. Thermodynamics taken very seriously.

Tires aren’t the only parts to which the science applies. Engines are an obvious focal point. They too have optimal temperature ranges, and high compression race engines generate an overabundance of heat. Brakes get hot enough to turn water instantly to steam, and they too have to dissipate enough heat to remain at an optimal performance level. Exotic materials are everywhere.

Exhaust systems are an outlier. While they have an optimal temperature, that range is essentially as hot as possible. It is desirable to retain heat in the exhaust system. Hot exhaust gas maintains its velocity, helping to evacuate the cylinders for the next charge. Cooling exhaust slows the process. Here again, exotic materials can help.

Titanium is one of those prized materials. It’s used in racing exhausts for the same reasons it has been used so extensively in supersonic aircraft. Most people know that it is light and (some grades) strong, but few realize that it is also prized for its low thermal conductivity. The latter makes it a very poor choice for any gun part that is in contact with the barrel. Titanium barrel nuts, for instance, are a new fad for ARs, but they’re a bad idea. They insulate the barrel right where it’s generating the most heat. Oh, and despite the goofy insistence of pop culture, titanium is not bullet proof.

Another exotic material made popular in racing is carbon fiber. It too is prized for its light weight, strength—it’s very rigid—and thermal properties. In exhaust systems, it is often used in the construction of mufflers. It’s a superb insulator, helping maintain exhaust core heat, with the added benefit that riders and passengers don’t scald their legs on exposed mufflers. This should be a tip-off that, despite its rigidity, it’s a poor choice for barrels.

There are companies making carbon fiber wrapped barrels. By most accounts the good ones shoot quite well. In .22LR applications, which generate relatively little heat and have very long life cycles, carbon fiber may be fine if you can stomach the hefty price tag. Center fire barrels are another story. High-powered cartridges can heat a barrel in just a few shots. Trapping that heat near the bore will surely decrease the lifespan of the barrel, not to mention raise chamber temperature and pressure over lengthy shot strings.

To be fair, the addition of certain exotic media to carbon fiber epoxy resins can increase its thermal conductivity, but if barrel makers are doing so, they are mum about it. There’s an easy test to find out though. Put several shots in quick succession through your high-powered rifle. A barrel with good thermal conductivity (steel is half decent) will get warm to the touch very quickly, though of course the thickness of the barrel will have an influence. If your carbon wrapped barrel never gets noticeably warm, that’s a clear indication that the carbon fiber is insulating rather than transferring.

If your machine cannot incorporate dynamic cooling systems, like a modern engine with its water jackets, radiator, fan and pump, then material choices become very important to thermal performance. It may surprise many to learn that water is a lousy coolant. Its thermal conductivity is extremely low compared to most metals. Air is even worse. It’s actually a terrific insulator. But air and water are both cheap and plentiful, and if you throw enough of them at a hot spot, they will carry away the heat.

Your gun barrel does not have the luxury of a dynamic cooling system. Most of the time it sits in pretty static air, which is a better insulator than conductor. Air cooled engines had a pretty good run in the 20th century, and small ones are still used today, but they rely heavily on the rapid movement of air past cooling fins. Seen any cooling fins or fans on guns recently?

The firearms industry does not evolve at the breathtaking pace of the motorcycle industry. The most popular modern sporting rifle design is over fifty years old. The science of barrel making has improved, but still, the standard answer to the questions of barrel rigidity and thermal performance is: more steel. The concept has its limits. Steel gets heavy and cumbersome, and it’s hardly the ultimate thermal conductor anyway. Early machine guns used water jackets for cooling, and the Tommy gun had cooling fins near its chamber.Since then dynamic cooling has been all but abandoned on small arms.

Barrel steel is pretty cheap, but you aren’t reading this because you want to find out about the cheapest way to build a gun. There are plenty of cheap guns out there. It’s a race to the bottom, if you haven’t noticed.

High performance has a price tag. But the good news is, we’ve already eliminated two of the more exotic materials as candidates. A high-performance barrel doesn’t have to be made of un-obtainium. There are common materials with twice the thermal conductivity of steel.

In Part II, we’ll talk about materials that do make sense, and debunk some more hype. It’ll be fun. Bring your hot tubbing partner.

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Interesting points and observations, and sounds like its gonna get a lot of conversation going, I will throw my 2 cents in for what its worth, looking at the evolution, and continual advances in materials, there's not a lot out there that can beat good ole steel for its cost effective, and resilient nature for firearms application.. are there a lot of really cool "super metals and composites out there? sure, but looking at the cost can make almost anyone turn green and run..

you cant make a barrel out of titanium and expect it to last too long as it tends to oxidize because it will react with oxygen and carbon at high temperature, well below its melting point, making it very hard and brittle. let alone the cost factor..

then the possibility of Nickel based alloys that can handle, and maintain stability at temps pushing 2000 degrees that incorporate chromium, cobalt, and rhenium in such practical applications such as turbine blades, but they cannot handle the pressure spikes in the chamber of most rifles. then the sticker shock of a 1000$ replacement barrel...

there are options for explosion welding dissimilar exotic metals into a monolithic structure. but I would hate to see a pricetag justifying the science behind that to make "the perfect barrel" .... I would hate to mortgage my house to buy a firearm...

mentioning water cooled weapons of the past, they really worked for their application at the time, but would we want to lug around a gallon can of water plumbed to our rifles hunting in the field for that 10 point buck?

I kinda look at as many variables in a situation and application.. cost per unit, manufacturing process, machineability, tooling, effective lifespan, cyclic life, marketability, reliability, raw material supply and ease of maintenance. for what its worth, on most all those, steel has worked for well over a 100 years, and still going. I have firearms that are that age, and still work perfect.. and with all the advancements in modern metallurgy, the modern steels are leaps and bounds better all the time.

Most people will never shoot their firearms until you get full catastrophic meltdown and failure of the metal at the molecular level, and if you do, I commend you for your effort! Steel is relatively cheap, last a long long time if practically maintained. Replacement parts are very cost effective, and can last thousands and thousands of cycles With modern coating, and processing like hot bluing, parkerizing, and meloniting, the surface is getting even easier to maintain. then, add chrome lining, Nickel boron, and RF85 treatments that permanently inbed into the metals and add exceptional lubricity, your life cycle are greatly increased, and friction heat is greatly reduced.. and I can still afford to buy a replacement barrel for 150$ and not have to pay 10,000$ for a hunting rifle.

The retail market is the sole driver for continual sales, a product will only sell for what the market will bare, and we wont even get into the cost of doing business, payroll, product volume to keep the doors open and the lights on... gotta turn volume and make a quality product that people can rely on, and be confident it will work every time.
Kind of the old "if it ain't broke, don't fix it" it really does take a long time to get the masses to come around to new hi-tec things and there's an exceptional cost to bringing that to fruition sometimes, and its a big risk. and if your pricing is so far out of the realm, no marketing and advertising will bring people around. How many "ultimate, cool, and "revolutionary" materials have lasted on almost any firearm out there, sure there have been a lot of upgrades, and some unique things, but have they ever made it to the mainstream and stayed..... practically no...other than advancements in polymers for frames grips and mags..

when you're making a product that the average person will shoot maybe a few hundred rounds a year thru, like an average hunting rifle, the barrel is already spec'd to probably last for at least 10,000 rounds, and the chamber is generally spec's to withstand almost double the max load for safety. so for what its worth, it "will last forever"...

now, when you get into the military world, or full auto , your round counts go up tremendously and longevity of parts are really strained because of heat and friction, but in our normal people world... if i shoot out my AR barrel, I can replace the barrel in about 20 minutes for between 100$ to 300$ depending on quality, and that's not bad...heck... one of my AR's has close to that thru it now, and it still shoots as accurate as ever...
 
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Discussion Starter #3 (Edited)
Good observations Flatsneck. Your points on cost and consumer driven price structure are well made. However, in keeping with my previous analogy, I have to point out that in the realm of high performance motorcycles, those arguments dissipate to some degree. Technology trickle-down is very rapid with sport bikes. Today's top-of-the-line street sport would have been a podium contender on many tracks a decade ago. In fact, Honda just announced availability of a street legal version of its 2014/2015 Moto GP Championship winning race bike! While that replica will surely carry a premium tag, prices of the company's flagship CBR series--which offers mind-numbing performance by any measure--have barely kept up with inflation. When I bought a new CBR900RR in late '99 (essentially last year's model) it was about $10,500. Today, a 2015 CBR1000RR will run you around $13,000 or so. There is a light year of performance development separating the two models.

Despite the CBR's obvious racing pedigree, you'd be hard pressed to find any exotic materials on one. Advances are accomplished through big R&D budgets, sound engineering, and economies of scale. I appreciate that. RPP spends virtually all of its discretionary budget on R&D, and we have no economy of scale. The big firearms manufacturers have just enjoyed a virtually unprecedented, decade long sales boom. Innovation would follow in most industries, but price reductions are just about the only headline in the firearms industry. I can't help but remark on it. The good news is, it leaves lots of room for companies like mine to make a living improving on the status quo. :cheers:

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Great read wish I was smart enough to contribute, but studied science not engineering. I have found out over my professional career scientists and engineers speak different languages. I do understand that technology and advancements in materials certainly should be looked at in regards to dissapating the heat of this one cylinder engine. I will stay tuned to this thread as I fear there are a bunch of engineers and red neck engineers on board this forum.
 

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All great points that I cannot argue with in regards to the motorcycle world.

This... "RPP spends virtually all of its discretionary budget on R&D, and we have no economy of scale. The big firearms manufacturers have just enjoyed a virtually unprecedented, decade long sales boom. Innovation would follow in most industries, but price reductions are just about the only headline in the firearms industry. I can't help but remark on it. The good news is, it leaves lots of room for companies like mine to make a living improving on the status quo."
I commend you for that!I'm a lot like you on that front, to a point..I do what i can to advance my products function and performance to be able to supply the best part i can to the public.... granted, I only make muzzle brakes exclusively. So my my product line is nowhere near as vast as yours.
I, as a little guy, have hours and hours of real world prototyping, testing, longevity testing and revising, then more testing, and re-designs and tweaking until i settled on something that I am confident about bringing to market and adding to my SKU's.

then I fall right into the weeds of competition from major name brand manufacturers with million dollar budgets for advertising, sponsor competitive shooters etc. I have direct comparison tested my products against everyone out there, and I know from my track record and customers that I have a solid product line that is as good, and in a lot of cases outperforms major manufacturers that sell their products for 5 times what i do, If i try to move at that price point, I would NEVER sell a single brake.. then on the bottom end, there's a flood of "ebay special "made in china" brakes flooding the market... I deal with "i can get a brake for 1/2 of what you charge, will you drop your prices to compete" my answer is an astounding NO... I'm good right in the middle....
Biggest thing where see the issues on advancing the envelope in the firearm world is directly related to cost sadly.. granted they have had record sales and a huge boom under the Obama cloud of doom.. and when your'e in the market field where you are vilified, and demonized by every zealot out there, I can see a little fear factor also mixed in with potential liabilities... and the amount of moneys going out for insurance, regulations, legal teams on high alert, and just being a competitor in a large field of options..

Spending millions and millions of dollars on trying to bring to market a new fancy whiz-bang idea-part for global sales is a daunting process that is very costly on the up front side..
you have a limited number of options, and they're all not in your favor.
1. Come up with something, and be the first, but costs will be very high because of lack of volume produced, and try and sell high to recoup, or break even, hopefully people will catch on..
2. sell at a loss to get the product out to market, then you'll have to eventually raise prices to actually make a profit.
3. defend your idea from others looking to jump in on it.. and possibly have to compete... (or pay TONS of money defending with copyright and patent application.)
4 sell item a loss as a "loss-leader" to get the item out there relying on profits from the rest of the companies income to offset the losses and float the project until pricing on materials come down by increasing your volume buying power..



Technology trickle-down is very rapid with sport bikes. Today's top-of-the-line street sport would have been a podium contender on many tracks a decade ago.Absolutely true!Firearms are a great platform for innovation, as new cool innovations trickle out every year...
but when you're dealing with motorcycles with the amount of parts, and technologies all combined to create a marvel of modern engineering to perform a task as technical and intricate as they do with every rev of the engine, (hundreds and hundreds of parts in perfect harmony) then relate to a firearm with 30-50 parts that only have to line up correctly and not bind.. and keep competitive with dozens of manufacturers competing for the same dollar....

 
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Discussion Starter #8
Let me ask this, wasn't it the intent back in the late 80's early 90's having a barrel cryogenically​ treated would change the molecular structure of the metal reducing the stresses of heat? Was there any truth to this or just BS?
My personal jury is still out on that question, as I haven't verified the claims. I did get a poor shooting barrel cryo treated 10 or 12 years ago. It didn't improve the barrel in any measurable way, but that's hardly a comprehensive test. We buy some cryo treated dovetail cutters from Brownells for the manual mill, because the cost difference is nominal over plain HSS. Are they better? Hard to say. We cut a lot of dovetails, and both types have a limited shelf life. We run carbide only on the CNC, and those, of course, outlast any steel cutter.

One can only accomplish so much with metal treatments. While there may be a difference between oven stress relieving and cryo, I suspect it's a small one. Premium barrel makers are in a competitive industry. A modest cost increase might be worth it to them to find an edge, but as far as I know cryo treating hasn't been widely adopted in place of traditional stress relieving.
 

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All kinds. Enamored of their mechanisms!
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Dimple, flute, and fins help dissipate heat. They can affect harmonics related to accuracy. Weight, size, cost, and complications get in the way of aesthetics.

Trust me on this - Our tax dollars are being spent on research to alleviate or at least mitigate heat build up in our soldier's firearms and suppressors.

AC
 

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gunscrewguy....
Trust me on this - Our tax dollars are being spent on research to alleviate or at least mitigate heat build up in our soldier's firearms and suppressors.
Seeing that our men & women soldiers have been sent to the Hell Holes of this planet for nearly 30 years now...I have no doubt of this at all...!!
 

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Trust me on this - Our tax dollars are being spent on research to alleviate or at least mitigate heat build up in our soldier's firearms and suppressors.

AC


Hey AC I agree and am proud to fund such research and hopefully this will trickle out to the folks like RPP, then to us.....or I'm hoping RPP solves the problem for them!

I have read a few articles on the explosion welding, while it is not new technology it intrigues me. I am wondering why it is not widley adapted? Is it expensive, different skills different tool or just not the answer to any firearms development? seems if you could bond dissimilar metals than this could be used in a creative way to dissipate heat? obviously I am not a metallurgist or an engineer.

oh and I never took thermodynamics in school but roomed with folks that did, so maybe some of the discussions sound familiar........
 
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Discussion Starter #12
Great read wish I was smart enough to contribute, but studied science not engineering. I have found out over my professional career scientists and engineers speak different languages.
Mrguvna, contributions to innovation come from strange and unexpected quarters. Orville and Wilbur Wright were bicycle mechanics. I forget what they engineered, but it was something big. Oh yeah, the first successful flying machine. It's true that engineers and scientists speak different languages, just as machinists see things differently than the engineers designing the parts they will machine.

Mark Twain said "Education is what's left when we've forgotten everything we learned in school." It's true enough for me. While I studied mechanical engineering, my first career was as a motorcycle mechanic. I commuted to and from campus every weekend on my bike, and because I was a poor college kid who could only afford an old bike that needed lots of maintenance, me and that machine got pretty intimate. Thus began a long love affair with motorcycles that pulled me into the pits instead of the office. Once I got past the idea that it wasn't a very glamorous job, I loved it, eventually becoming one of the most successful in the industry. My engineering education afforded me a critical eye of engine and chassis architecture, but more importantly, my experience as a mechanic and avid rider afforded me a highly critical perspective of the engineering involved. I could spot engineering mistakes after 5 minutes in the saddle, or an hour under the wrench. I also developed an uncommon ability to understand mechanical systems at a glance.

By the time I left the industry in 2004, motorcycles had been refined so much at the factory level that aftermarket improvements were getting much harder to come by. But my appreciation of firearms had already led me to amateur gunsmithing, and in that industry I spotted all kinds of room for improvement. By the time I apprenticed I was teaching my would-be mentor more than he was teaching me. So here I am, an engineering student turned mechanic, turned gunsmith/engineer/designer/machinist, not quite speaking the same language as anybody. Or probably speaking just enough of each language to get lost right when the conversation turns really interesting. What the heck, I'm having a ball.

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I have always wondered why the Mauser 98 has been hailed as the pinnacle of rifle development that other manufactures strive to achieve. I read that over and over in gun rags every year. Adam, I'd love to be invisible and ride your shoulder for a couple of months! I'm honored and proud to call John Linebaugh a friend, I'm gonna go sweep his shop soon to see what I can learn while there. Love to do the same with you. DP
 
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Discussion Starter #15
I have always wondered why the Mauser 98 has been hailed as the pinnacle of rifle development that other manufactures strive to achieve. I read that over and over in gun rags every year. Adam, I'd love to be invisible and ride your shoulder for a couple of months! I'm honored and proud to call John Linebaugh a friend, I'm gonna go sweep his shop soon to see what I can learn while there. Love to do the same with you. DP
Thanks for the compliment DPE. I find my job pretty interesting, and sometimes even exciting, but I have no idea what it looks like to an outsider. I suspect that most of the fireworks are in my head.
Pretty cool that you know Mr. Linebaugh. I'm a little afraid of his beastly handgun cartridges, but do tell him thanks, next time you see him, for all the .44 Ripsaw brass. :smokin:
 

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Back to he source. Tell me. What exactly generates the heat? Burning of propellant or friction from the piston, sorry, bullet. How much is the heat generated altered, if at all, by a lower turn rate? Or a different type of rifling, say micro groove to ballard (this is a Marlin forum). And, as heat must be generated, short of water cooling and barrel changes required for military sustained fire use, what innovative options have we to re direct or dissipate the heat?

Your motorcycles have one advantage guns can never have, a considerable airflow. Whether fins or a radiator, you need air, lots of it. (Suzuki 500cc World Champion Team engineering dept. Croydon England. 1976 I think, Barry Sheen riding, some great times).
 
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Discussion Starter #17
Tranteruk,

Good questions. Some are a bit beyond the intended scope of my discussion, but I'll do my best to answer them. Others will (hopefully) be answered to your satisfaction in Part II. Before I jump in though, let me just say I'm pretty sure you have some stories I'd like to hear.

Modern smokeless propellants burn at up to 3400 degrees F. So every time you pull the trigger, it's roughly the equivalent of firing a propane torch through the bore. The combustion event is quick, typically around 1ms (.001 second) depending on cartridge and barrel length, but it's a lot of flame heat nonetheless. Friction will join the party too, raising the bore's surface temperature substantially (please don't make me do that math--it's early). The actual amount will vary considerably with bullet length, construction, and composition. Barrel bores don't have the advantage of an engine cylinder's constant lubrication. Jacketed bullets will add more heat from friction than lubed hard cast boolits. Suffice to say that firing either will get you lots of heat in the bore from the combustion event alone.

Twist rates can make a difference, though again, the math to make that determination lies somewhere between my second cup of coffee and the end of my life. It does take more energy to accelerate an object in two vectors than it does in one. On motorcycles, shaving a couple pounds off the wheel/tire weight can offer a significant performance boost. Wheels must be accelerated in rotation as well as the direction of travel. Similarly, good riders know that in order to maintain a given corner entry speed, the throttle opening must be increased through the turn. Put another way, if your Mini runs out of gas--er, petrol--it's relatively easy to push in a straight line. Turn the wheel hard left though, and you may be trying to flag down another motorist for help. Hopefully that good samaritan will be Rebecca Ferguson. She may not assist you in actually pushing, but you're sure to find a reserve of strength.

At any rate, the additional energy needed to spin a bullet faster will manifest as extra heat in the bore, especially in the region of the throat, as much of the action takes place in the first inch of bore. Combine this with the fact that faster twist rates are commonly employed to spin up longer, heavier bullets, and the additional heat generated can be significant.

On the question of microgroove vs traditional rifling, I suspect that's going to be a wash. All other things being equal, friction in the bore will vary depending on surface contact area. With MG lands being about .0025" tall, and traditional lands being around .005-.006" tall, six groove traditional rifling will expose roughly the same amount of surface area to the bullet jacket as 12 groove micro will.

The important take-away is that there is no getting around what happens in the bore. All barrels are going to build up heat, to a greater or lesser degree, based on a range of factors, but they will all suffer from it. Higher bore/chamber temps will affect combustion pressures, resulting in vertical POI shift. High temps will also bring inherent barrel stresses into play, causing corresponding POI shifts as the barrel "walks." Even if walking is minimized through proper stress relief techniques, the heating barrel will expand, slightly increasing bore diameter, another variable come to play. It is also understood that as bore temps get very high the barrel's resistance to wear can decrease, reducing the barrel's useful life.

None of the above is good, so let's get the heat out after we get the lead out.
 
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Discussion Starter #18 (Edited)
This is a topic that I've been thinking about for some time. Have some design options in the works but thought I'd share with all of you smart MOF friends to start what I hope will be some informative and interesting dialog.

Performance Firearms and Thermodynamics. Part I
Unless you work as an engineer, you probably haven’t given much thought to thermodynamics since your last physics class. If you bring up the topic whilst hot tubbing, people will start putting their clothes back on. It is a fascinating subject, but perhaps a little dry for hot tubbing. In industries focused on performance machines, thermodynamics is a vital science. If you’re in it to win it, ignorance of the subject will result in your complete annihilation.


Having spent 15 years in the high performance motorcycle industry, I am now puzzled by the reality that the firearms industry has neglected this science for so long. After all, two truths are, or should be, understood by now: 1) firearms generate a lot of heat; and 2) heat is a barrel’s worst enemy. So why has so little attention been given to disposing of that heat? To make matters worse, the industry itself often misinforms consumers and muddies the waters. But I’ll get back to that. For now, let me illuminate some basic principles, drawing on my background in high performance motorcycles. Guns are essentially single stroke engines, so there are plenty of analogies.

Compared to the firearms industry, motorcycles evolve at a breathtaking pace. It wasn’t always that way, but these days there is an enormous amount of money at stake. Honda, far and away the global leader, sells roughly 15 million units annually. They and their competitors understand that success on the race track sells bikes. So these are very serious battles, not just by the riders, but by the hundreds of engineers and support staff behind the riders. The most serious arena of all is Moto GP, the premier class, equivalent to F1 in auto racing. Factories will spend hundreds of millions to compete, and find some technological edge, at this level. And not just motorcycle factories either.

Tire companies like Bridgestone and Pirelli will also spend millions in development to ensure that their tires are on the winning bikes.Tire engineers study thermodynamics a lot. They are the unsung heroes of the sport. The best engine, chassis, and rider will get nowhere near the podium without a reliable way to transmit all that power to the tarmac. And all that power transmission to the tarmac generates–you guessed it–lots of heat. Tires have an optimal operating temperature. Too cold and the rubber compound doesn’t deform well enough to grip the road surface. Too hot and the compound can become “greasy” and unpredictable, or deteriorate too rapidly to keep the rider in the race. Tire warming jackets are used in the pits to ensure that the optimal temperature is achieved before the bike hits the track. Thermodynamics taken very seriously.

Tires aren’t the only parts to which the science applies. Engines are an obvious focal point. They too have optimal temperature ranges, and high compression race engines generate an overabundance of heat. Brakes get hot enough to turn water instantly to steam, and they too have to dissipate enough heat to remain at an optimal performance level. Exotic materials are everywhere.

Exhaust systems are an outlier. While they have an optimal temperature, that range is essentially as hot as possible. It is desirable to retain heat in the exhaust system. Hot exhaust gas maintains its velocity, helping to evacuate the cylinders for the next charge. Cooling exhaust slows the process. Here again, exotic materials can help.

Titanium is one of those prized materials. It’s used in racing exhausts for the same reasons it has been used so extensively in supersonic aircraft. Most people know that it is light and (some grades) strong, but few realize that it is also prized for its low thermal conductivity. The latter makes it a very poor choice for any gun part that is in contact with the barrel. Titanium barrel nuts, for instance, are a new fad for ARs, but they’re a bad idea. They insulate the barrel right where it’s generating the most heat. Oh, and despite the goofy insistence of pop culture, titanium is not bullet proof.

Another exotic material made popular in racing is carbon fiber. It too is prized for its light weight, strength–it’s very rigid–and thermal properties. In exhaust systems, it is often used in the construction of mufflers. It’s a superb insulator, helping maintain exhaust core heat, with the added benefit that riders and passengers don’t scald their legs on exposed mufflers. This should be a tip-off that, despite its rigidity, it’s a poor choice for barrels.

There are companies making carbon fiber wrapped barrels. By most accounts the good ones shoot quite well. In .22LR applications, which generate relatively little heat and have very long life cycles, carbon fiber may be fine if you can stomach the hefty price tag. Center fire barrels are another story. High-powered cartridges can heat a barrel in just a few shots. Trapping that heat near the bore will surely decrease the lifespan of the barrel, not to mention raise chamber temperature and pressure over lengthy shot strings.

To be fair, the addition of certain exotic media to carbon fiber epoxy resins can increase its thermal conductivity, but if barrel makers are doing so, they are mum about it. There’s an easy test to find out though. Put several shots in quick succession through your high-powered rifle. A barrel with good thermal conductivity (steel is half decent) will get warm to the touch very quickly, though of course the thickness of the barrel will have an influence. If your carbon wrapped barrel never gets noticeably warm, that’s a clear indication that the carbon fiber is insulating rather than transferring.

If your machine cannot incorporate dynamic cooling systems, like a modern engine with its water jackets, radiator, fan and pump, then material choices become very important to thermal performance. It may surprise many to learn that water is a lousy coolant. Its thermal conductivity is extremely low compared to most metals. Air is even worse. It’s actually a terrific insulator. But air and water are both cheap and plentiful, and if you throw enough of them at a hot spot, they will carry away the heat.

Your gun barrel does not have the luxury of a dynamic cooling system. Most of the time it sits in pretty static air, which is a better insulator than conductor. Air cooled engines had a pretty good run in the 20th century, and small ones are still used today, but they rely heavily on the rapid movement of air past cooling fins. Seen any cooling fins or fans on guns recently?

The firearms industry does not evolve at the breathtaking pace of the motorcycle industry. The most popular modern sporting rifle design is over fifty years old. The science of barrel making has improved, but still, the standard answer to the questions of barrel rigidity and thermal performance is: more steel. The concept has its limits. Steel gets heavy and cumbersome, and it’s hardly the ultimate thermal conductor anyway. Early machine guns used water jackets for cooling, and the Tommy gun had cooling fins near its chamber.Since then dynamic cooling has been all but abandoned on small arms.

Barrel steel is pretty cheap, but you aren’t reading this because you want to find out about the cheapest way to build a gun. There are plenty of cheap guns out there. It’s a race to the bottom, if you haven’t noticed.

High performance has a price tag. But the good news is, we’ve already eliminated two of the more exotic materials as candidates. A high-performance barrel doesn’t have to be made of un-obtainium. There are common materials with twice the thermal conductivity of steel.

In Part II, we’ll talk about materials that do make sense, and debunk some more hype. It’ll be fun. Bring your hot tubbing partner.

View attachment 464618
Performance Firearms and Thermodynamics: Part II
In Part I of this article I talked a little bit about materials and their thermal conductivity. The gist of that first piece was to illustrate the importance of material choices in high performance machines–among which we would like to count firearms–where good thermal management is critical.

Does it even matter to the average shooter? For target shooters and competitors, I’d say it matters. For someone like a military sniper, who may need to place a long string of shots with extreme precision, I’d say it matters a lot. For hunters, who only need one or two predictable shots, maybe it doesn’t matter much. But maybe the peace of mind is still worth something.

It has been my experience that many mass produced hunting rifles--bolt actions included--will give ambiguous results during zeroing. Assuming four shots with a scope adjustment, it's not unusual to have a three shot group of 2 inches, give or take a little. If that's adequate to your task, then you can pack up. If you must know with more certainty where that first cold bore shot will go, you'll have to wait half an hour or more for the barrel to cool completely. A light weight barrel that walked less and cooled much faster would be worth something to me. My time is valuable.

I mentioned in Part I that traditional coolants like air and water actually have pretty lousy thermal conductivity compared to most metals. We need to revisit air, because it plays a decisive role in our “static” firearm cooling systems. First, a bit of science speak, and then we’ll be off to the races again.

Thermal conductivity is measured in more than one set of units, but in the SI system it is expressed as watts per meter-kelvin. For our purposes, understanding the expression is unimportant, other than to note that it affords us a useful means of comparison between materials, so I will drop its use immediately. But to give you a sense of scale, here are two materials at opposite ends: diamond, at the very highest end of the scale, has a thermal conductivity of 1000, while at the lower end of the scale blown glass insulation has a mere .04. So diamonds transfer heat 25,000 times faster than insulation. Bet your wife didn't know that. :biggrin:

(CHART: Thermal Conductivity of Common Materials)

While we're comparing, here are a couple of numbers for the exotic materials I mentioned in Part I: titanium has a 22 rate. Carbon fiber can vary greatly depending the resin used and the arrangement of fibers and fillers, but the low end (traditional resins and fiber planes) is about 7 and the high end is around 150. Not bad on the high end, but still nowhere near as good as much cheaper aluminum alloys which can get into the low 200s, and whose thermal performance increases with temperature.

So where does air, our precious barrel coolant, fall in the scale? Get ready for disappointment. Depending on atmospheric conditions, static air is about .02-.024. Worse than most materials sold as insulation! In fact, most insulating materials are little more than a means of creating dead air space, because dead air is a superb insulator.

To be fair, air is hardly ever static in our world. Like most fluids, it moves in currents, thanks in large part to temperature gradients. But it is disturbing nonetheless to realize that the one coolant firearms can rely on is actually one of the world’s worst. As we talk about thermodynamics in gun barrels it’s important to remember that air is effectively a barrier, unless we can move large quantities of it.
This is one of the reasons that rifles competing in accuracy have very thick barrels. Compared to air, steel is a very good thermal conductor, with some alloys rated in the 80s. Since heat moves across gradients, more steel creates a larger gradient, effectively “soaking up” heat at the hot bore and moving it outward toward the surface of the barrel, where it meets–ugh–air, coming to a screeching halt. Once the gradient ceases to exist, ie the barrel’s temp is normalized, it is up to the tediously slow process of air cooling to bring down the temp of the barrel.

Fire five shots through your rifle in quick succession, and you will feel the exterior of the barrel get warm to the touch in just seconds, thanks to the fast acting thermal transfer of steel. Now wait for the barrel to return to “cold.” It can take 30 or 40 minutes. Because air sucks. If this was not the case, your barrel would cool as rapidly as it heated. Look at it this way: in terms of thermal gradients, your barrel steel is a late summer brush fire. By comparison air is a thin, patchy lawn.

In light of this we must consider a means of speeding the transfer of barrel heat to ambient air. Luckily, we do get a little help. A warm barrel creates its own thermal currents in the surrounding air. As we know, warm air rises, so the heated air right next to the barrel’s surface will rise, setting up a current that curls lazily around the barrel, drawing fresh, cool air from beneath. The larger the gradient, the stronger this current will get, resulting in more rapid cooling from high barrel temps. But at smaller gradients this current is paltry.

Barrel fluting has become pretty popular these days. Does it help? Let's look at an example. I have a fluted DPMS AR-10 barrel (take-off) that will serve our purposes. This barrel is 1.05" under the hand guard, with six .25" flutes cut for 7" longitudinally. We'll skip the middle school math and go straight to the results. These six flutes, cut .125" deep, expose 6 square inches of additional surface area on the barrel. That sounds pretty good. But the barrel's total surface area without flutes is 52 square inches. The increase due to fluting is about 11.5%. Not very impressive. Air cooled cylinder heads have fins that double or triple their surface area, and without forced airflow they will still overheat.

Another thing to contemplate: The material removed during fluting is steel, having a conductivity rate of roughly 80. In its place is now air, with a conductivity rate of .024. Hmm. That extra steel could have helped normalize your barrel at a slightly lower temp than the barrel whose mass has now been reduced by fluting. Also, flutes are nearly always cut longitudinally, but hot barrels do not create longitudinal air flow. They create vertically rising currents, and longitudinal flutes can effectively become dead pools of air, like eddies in a stream. Oops. You’ve traded a good thermal conductor for pretty little pockets of insulation.

It’s true that flutes reduce barrel weight. In our example barrel the reduction is about 4.5oz. It’s also true that flutes do not reduce rigidity quite as much as turning an equal amount of mass from the barrel’s whole diameter. But alas, it is also true that flutes introduce machining stresses into the barrel that otherwise would not have been there. These stresses can raise their ugly head at surprisingly modest temperatures, often limiting, rather than increasing the number of shots in an accurate string as the barrel “walks” earlier. It’s for all these reasons that some premium barrel makers will not flute their barrels. If you must have a fluted barrel, be sure that it has been stress relieved after fluting. This will generally not be the case with mass produced barrels.

So what can we do with all that heat generated in the bore, without simply adding more and more steel to the barrel? The options are few, and without a dynamic cooling system a barrel will always reach a temperature saturation point limited by its mass. I for one am not ready to rule out dynamic cooling for small arms.

Liquid cooling is a non starter. While water transfers heat 26 times faster than air, it is still far worse than most metals. Without circulation and a heat exchanger it will simply be a barrier to heat transfer. There are materials far superior to liquid that can pull heat away from the hot bore. Aluminum, for instance, conducts heat more than twice as fast as steel. Copper has a phenomenal 400 conductivity rate.

Copper's other properties make it a poor choice for barrel use, but aluminum, in combination with traditional barrel steel, could make a lot of sense. This avenue has its own detours, like differing expansion rates, harmonics, and extra machining, but a well engineered, cost effective solution could have real merit. Aluminum is quite rigid, exceptionally light, affordable, and can pull heat away from the bore very rapidly, potentially increasing barrel life and accuracy at once.

Once the heat has been whisked away to the barrel's surface, we still need a means of dissipating it. Air sucks, but it's all we've got. Let's move it. Semi automatics use combustion energy to operate mechanical systems. Why can't one be a forced air system? Even a little forced air would be a big improvement over the modest thermals created by the hot barrel. And, if we force the air along fins that seriously increase surface area, then we've got an actual cooling system.

There are challenges inherent in such solutions, and nothing will be as cheap as a basic chro-moly barrel. But I believe good engineering and modern manufacturing methods can incorporate some real improvements without exotic pricing. Doubtless, early adopters can expect to pay a premium price, but with consumer awareness and sales come economies of scale. There could be a high performance barrel in your future.

Adam
 
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