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Discussion Starter · #1 ·
It seems like it is rather easy, at times, to become a bit careless about the way we regard pressures. I say "we" because I know I have been guilty at times, but also because there tends to be post of various kinds in various places regarding ways to squeeze just a bit more out of the cartridges we use.

It is fortunate that most of the arms we have to work with were designed to handle excess pressures over and above their working pressure. For some that margin of safety may seem like an excessively safe working margin, perhaps built in at the insistence of overly cautious legal staff. But is it?

A couple of documents have come through my hands recently which point out the dangers of being too casual or becoming to stuck to certain "facts." The June issue of Handloader had an article on chamber pressure by John Barsness. As many know he has often referred to the chronograph as being as good an indicator as anything short of actual pressure testing equipment when used in conjunction with pressure tested data for determining safe loads. And while he doesn't declare this to be gospel I think it is easy to misapply the idea as gospel. In the stated article he points out how the changes in a single component, the primer, resulted in huge pressure differences while showing insignificant changes in velocities.

Primer "A" showed a pressure of 63,800 psi with a velocity of 2964 fps.
Primer "B" showed a pressure of 55,800 psi with a velocity of 2920 fps.
Primer "C" showed a pressure of 70,100 psi with a velocity of 2991 fps.

All components were identical except for the primer and the pressures were obtained via the strain gauge method.

Another problem is the pressure measuring methods and what they mean. While many folks seem to be throughly cognizent of the fact that crusher pressures (CUP) and piezo pressures (psi) are no more than loosely correlated, there seem to be some who want to extrapolate or interpolate data using one or the other. I came across another published source, an Alliant Reloader's Guide, which listed some of the CUP/psi comparisons. The results are enlightening in the fact that they are clearly not interchangeable.

For example at 52,000 CUP the 223 is equivalent to 55K psi, the 243 and 308 are equivalent to 60K psi, and the 6mm and 270 are equivalent to 65K psi.

The bottom line, I suppose, is to to be careful and use data exactly especially when approaching the top ends. That is nothing new to many folks and I am only "preaching to the choir" to you if you are but it may be advice that some of the folks new to the craft of reloading have missed.
 

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Golsovia: Thanks, that information is very useful, and I will keep it in mind when working up loads. The amount of the pressure differences with the different primers is surprising. I don't have a copy of the Handloader article by Barsness. Did those pressure differences result from using large rifle primers vs. magnum primers; or were they the result of using different brands of large rifle primers?
 

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Discussion Starter · #3 ·
The loads were built as 300 Win Mag cartridges using Winchester cases, 180 grain Nosler Partitions, 75 grains H4831, and three different brands and types of primers: Fed 215M, CCI BR2, and WLRM. I didn't identify them in the first post since it really isn't relevant to my point. While one might assume that the WLRM (C) primer will give higher pressures (and might consequently be substituted for at random if it is used in published data) that assumption would be erroneous without data to support it. We can't know, based on what was provided, if this particular primer raised pressures with just this particular combination of components or if it is typical across the board.
 

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It's a basic we can all live by......whenever a component is changed we should begin the working up process all over again. Some will argue that changing cases doesnt matter much but if you are at near max levels it could be deadly.



Perferator
 

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Perferator said:
It's a basic we can all live by......whenever a component is changed we should begin the working up process all over again. Some will argue that changing cases doesnt matter much but if you are at near max levels it could be deadly.



Perferator
Well said especially on the cases.Once you take data with a certain case-bullet and data and then use a smaller capacity case and a longer bullet...Well..Thanks to the proof of these cartridges for people that do that...Know one has got hurt.Size does matter in reloading.

Good post Goslovia...Jayco
 

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good stuff. the primer illustration has been shown many times before but I still wonder if folks are getting it. Life is good, BestLever
 

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Assuming the 63,800 psi load was a published maximum load within safe operating parameters using Primer "A", and its velocity was close to the published maximum velocity, if one were to change the primer from that load to Primer "C" and work up, I wonder what the pressure would be with that primer and a velocity of 2964 fps? I think it is likely, that while one might still be a little too hot, the practice of reducing starting loads and working up when changing components, and not exceeding the velocity of a published load, would prevent the prudent reloader from reaching the too hot load indicated by Primer "C".

For what it's worth, the difference between the assumed safe maximum load of 63,800 psi and the too hot load of 70,100 psi is 9.8%. That difference in the Marlin 1895 would put a too hot load due to primer differences at about 43,900 CUP. Certainly too hot for my tastes, but nothing that portends impending doom for the rifle or shooter.

I think if accepted reloading practices are followed this condition can largely be avoided. However, due to the cost and performance of primers I would see little need to change the primer from that of a published load.

Never exceed either the charge or velocity of a published load.
When changing components of a published load, reduce the starting charge by 5% and work up carefully toward the maximum charge watching for signs of excessive pressure or velocity.
 

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Discussion Starter · #8 ·
As usual, jackfish, a well-thought analysis of the original thread especially as it relates to the numbers.

To be honest, I'm not really sure what to think. On the one hand I want to go with what I know is the extra strength of my modern weapons; on the other, I know that extra strength is there for safety.

Then again, when using pressure tested data it would seem that enough of the variables have been covered that we are okay if we follow the recipes exactly -(or as exactly as we can considering that we know nothing of the component lot numbers, or several other variables we can't control.)

Hodgdon has published pressure tested data for some time (as have others). Hodgdon has only been specific about the components used as recently as their #27 manual. The #26 was no more specific than primer size (not brand), approximate bullet weight (not specific bullets), etc. , yet they show pressures they got. Is that data better - or worse- than what they show in #27? I don't know if there is a absolute answer to that. One thing seems certain, we are getting more information about the pressures we are using. We have the ability to shoot safer handloads than we have in the past. I suspect we still have a ways to go in terms of knowing the pressures we work with. Until then, staying out of the built-in margin of strength seems like the prudent thing to do - at least to me.
 

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Hello Riflemen-good stuff for any reloader as far as I'm concerned and very important information for handloaders new to the hobby. One thing I will add to this thread that is not always thought about by handloaders is presure stress over time.

Max. presure including built in saftey factor for 1 shot

" " " " " " " repeated shots

A hot round may work wonderful 11 times and then the barrel under repeated stress gives up on #12. This is one of the reasons for the built in saftey factor. To handle repeated upper limit published loads. Life is good, BestLever
 

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I was going over pressures in Lee's Modern Reloading book, and it's pretty hard to make sense of what they put into the tables... Some loads are listed in P.S.I. , others are listed in C.U.P. , and yet others are listed in C.I.P. ???? How to make sense of this pressure data? I'm slightly at a loss here :?

Regards,

Doc Sharptail
 
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