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Edit: .223 Remington to 5.56mm NATO

OK, I'm putting this out there. The throat is considerably longer in a 5.56mm NATO chamber than a .223 SAAMI chamber. A steady diet if 5.56 puts unnecessary strain on the barrel extension and bolt heads of ARs. Pressure can rise >15,000 psi over SAAMI. Shoving a 5.56 NATO reamer will work but unless great care is taken it is possible to cut the shoulder in the process thus increasing headspace.

I have a design in mind that will eliminate such a mistake. It may not sell in quantities as it will wipe chrome out in the throat area on chrome lined barrels. For those who don't care, or have guns who's barrels are not chrome lined it will work with just a few turns with a T-Handle and rinsing out the chips.

This will also increase the distance to the leade to when the bullet engages the rifling possibly affecting accuracy and reducing chamber pressure.

Comments, please.

AC
 

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I like the idea. I want more info. Many an AR have passed through here....some with .223W chambers and some with 5.56....none with .223 only (who makes one?).
 
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Discussion Starter · #4 · (Edited)
My Reply from Clymer:


Way ahead of you!

Michiguns Ltd. - AR-15 Tools and Accessories

We make these reamers in production for these folks…but do not stock them…just sell them directly to m-guns….

Sincerely,

Staff

1605 West Hamlin Road
Rochester Hills, MI 48309
248-853-5555
CLYMER Precision
www.clymertool.com





Michiguns Ltd. - AR-15 Tools and Accessories

We make these reamers in production for these folks…but do not stock them…just sell them directly to m-guns….

Sincerely,

Staff
CLYMER Precision
1605 West Hamlin Road
Rochester Hills, MI 48309
248-853-5555


www.clymertool.com

At over $200 I may just rent or borrow a 5.56mm NATO reamer and proceed verrry carefully.

AC
 

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The reamer will get dull very fast trying to cut chrome. Just get a 223 Wylde chambered barrel and be able to shoot both 5.56 and 223.
 

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Discussion Starter · #6 ·
The reamer will get dull very fast trying to cut chrome. Just get a 223 Wylde chambered barrel and be able to shoot both 5.56 and 223.
You are correct on advising against reaming a chromed barrel. I would not ream a chrome lined chamber. Many chromed barrels are done poorly. Once the chrome is compromised, on a iffy chrome job, it will flake off like a cheap paint job. I was looking for an option to open up a commercial non-chromed bore.

AC
 

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How do you intend to deal with the chamber pressure difference?
After the chamber is reamed to provide the correct radial neck clearance, correct ball seat, leade length and angle .223 Remington ammo will drop in velocity and pressure, when fired in the NATO chamber, but the NATO ammo will give normal performance within specs., still higher than SAAMI .223 Remington, but within the design limits for an AR platform.
 
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Discussion Starter · #11 ·
I'll just borrow a 5.56 NATO reamer and be vewy, vewy, careful. It's for my #3 Ruger in .223 that I stole for $125.
 

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I'll just borrow a 5.56 NATO reamer and be vewy, vewy, careful. It's for my #3 Ruger in .223 that I stole for $125.
There are several different configurations of the NATO chamber. The chamber used in the M4 and M16A2 is different from the earlier chamber used in the M16A1, and the M4/M16A2 chamber differs a bit from the NATO STANAG drawing, which shows an 11 degree, 30 minute blend between the 23 degree shoulder and the case neck. This was needed for the M249 machinegun firing the M856 tracer because the base protrudes far below the chamber neck, and there were issues with base upset in which the bullet base upset into the shoulder area, sort of a "nailhead" effect, and pressure spikes of the expanded base locking into the shoulder blew guns apart. The Vom-Hoffe type double-angle shoulder supposedly eases the upset base being extruded down again and letting it exit the neck.
 

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Discussion Starter · #13 ·
Outpost75.

FN named it for a feature that is as much related to it's construction as it is for it's reducing pressure related to the SS-109 NATO round.

I am intimately familiar with the Forged FN barrel. Not only is the rifling cold hammer forged into a gun drilled blank, but the chamber and raw profile are finished in one forging operation as well. Using the right steel and FN's proprietary manufacturing processes it is a superior barrel.

The 11 degree shoulder angle (looks more like a radius with a borescope) is actually an artifact of the forging process. The forging process could not form the metal to that angle the meets the case mouth. My guess is they discovered the pressure benefit with extremely long bullets by accident. I know of one company's QC Dept rejecting their blanks because the 11 degree angle was not on the print. They later realized that they were actually a treasure trove.

Reamers are now available to produce cut chambers in the FN 'Dual Angle' chamber configuration.

I believe that it is a superior, do all chamber, for a working field gun. I might just get a hold of one of those reamers when I do my Ruger!

AC
 

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Gunscrewguy, Bravo Zulu! Thanks for the history which I wasn't aware of.

If you would happen to have the government drawing number for that chamber, that might be helpful for people wanting to order a reamer. I expect that JGS probably has the print on file, as we ordered TiCN coated production reamers from them having that feature.

During the Modified Operational Testing of the M16A1E1 at Quantico we took apart a couple rifles firing the 68-grain Colt-Federal XM287 and early production XM855 and XM856 from LC. NWSC Crane and Picatinny both examined the rifles and retested the ammo and found odd pressure excursions using the early BRL Minihat type transducer which weren't apparent using radial copper. They commented upon having previously observed similar blowups with the Colt CMG2 and Stoner LMGs using the 77-grain NWM heavy-ball ammo tested back in the late 1960s. Dr. Robert L. McCoy at BRL reviewed both reports and confirmed the failure mechanism. Of course these were not hammer forged barrels so didn't have the beneficial double-angle blend.
 
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Discussion Starter · #15 ·
If you would happen to have the government drawing number for that chamber, that might be helpful for people wanting to order a reamer. I expect that JGS probably has the print on file, as we ordered TiCN coated production reamers from them having that feature.
_______________________________________________________________

I'll root around but I have never seen the FN Drawings. I will talk with my Gurus. There are drawings of just the feature online. As far as I know the rest of the chamber is the same. I have access to a Wylde reamer and am hoping to do it this weekend. Should go quickly as very little material needs to be removed.

AC
 

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I wore out my 223 Bushmaster barrel after 3000 rounds of green tip 5.56. Nothing ever broke, rapid fire just fried the throat. New bbl has 1000+ with green tips and all original guts. Solution in search of a problem ?
 

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I wore out my 223 Bushmaster barrel after 3000 rounds of green tip 5.56. Nothing ever broke, rapid fire just fried the throat. New bbl has 1000+ with green tips and all original guts. Solution in search of a problem ?
Commercial barrels generally are not made to the same metallurgical specs as military barrels. M4 barrels are hammer-forged from a chrome-moly-vanadium alloy having a surface hardness of Rc32 min and with a surface finish of 16 microinch RMS, which is then electropolished and hard chromed. Such a barrel will stand up to full auto fire and should still pass a "stripping test" in which the barrel is heated first with 30 rounds of semi-auto, and then 30 rounds of 3-shot bursts, within a minute, then a 30-shot continuous burst is fired off a test stand through a witness target and there must be no wild, keyholed shots on the witness card.

Military accuracy requirements are also more generous and a dispersion of 1 mil is acceptable as long as no keyholes or wild shots are noted on the yaw card.
 
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