Max chamber pressure

[ronl]

Does anyone know the max chamber pressure for AR-10 type rifles? Trying to design a good load for my CMMG Mk3 CBR.

[steve4102]

The 308 Win. has a Max Average Pressure (MAP) of 62,000 psi.

http://www.saami.org/specifications_...essure_CfR.pdf

[Unclenick]

Correct. It's chambered in 308 Winchester/7.62 NATO. Probably the easiest pressure spec to understand is the CIP/NATO channel transducer pressure standard. For these cartridge names, the spec calls for a 10-shot average of peak pressures (Maximum Average Pressure; MAP) not to exceed 4150 Bar (60191 psi), with no individual round that contributed to that ten shot average exceeding 4772.5 bar (69219 psi), or a factor of 1.15 larger (+15%). Proof loads in the CIP system are 125% of MAP, rounded to nearest 10 bar, so 5190 bar, average, or 75,275 psi for the 308 Win. But CIP proofs are required to be repeated during a gun's life so it can be resold as being in working condition. SAAMI proofs are a one-time event for most guns being tested, and are higher.

The SAAMI spec for the conformal type transducer calls for a 10-shot average peak pressure, or MAP, not to exceed 62,000 psi, with an extreme spread among individual rounds contributing to that average not to exceed 12,800 psi. In theory, if nine of the rounds were exactly at 60,720 psi, the tenth could be at 73,520 psi and still satisfy this requirement. That works out to a worst case factor of 1.186, or +18.6%. The proof load range in the SAAMI system is 83,000-92,000 psi, so that upper limit is not quite there, but it's pretty warm and not very likely to occur in a real 10 round average. A real average would be expected to have no rounds exceeding 66,960 psi 97.7% of the time, as 4% is the SAAMI maximum allowed standard deviation of pressure in a MAP determination (10 round peak pressure test). At that standard deviation, only one round in 590,000 would be expected to reach the 73,520 psi theoretical limit, and that's only with 4% standard deviation. Most ammo has less than that.

Note that MAPs apply only to newly loaded ammunition. SAAMI has an allowance of an additional 4,000 psi for the 308 Win 10-shot average peak pressure after aging.

[Unclenick]

It seems to me I may have missed addressing the exact question the OP had.

What pressure the gun itself might be able to withstand: it should withstand the SAAMI maximum proof pressure (92,000 psi) at least once, though continued firing of those high pressure loads will tend to beat the gun up because its gas system is not designed to handle them on a regular basis. If you made the gas port smaller, you could theoretically tolerate more pressure until either the brass fails or the gun does. Very often, the brass goes first, making the failure pressure dependent on the make of the brass. Very often, military brass does best at this. Throat life is greatly shortened by regularly using increased pressures, though.

You could ask board member Clark if he ever had occasion to test this weapon to failure, as he has many others.

[Slamfire]

Unclenick spent a lot of time and effort explaining the design and theory, and did a good job. I am going to say, if you are looking for the maximum load the structure can withstand, you are going down the wrong path. Unclenick was just touching what I think the real limiter is for these guns: the gas system. Don't even think maximum loads in a gas gun. A bud of mine who has a Rock River AR10 went to the manual and picked out a close to maximum load for the thing and we spent quality time knocking fired cases out of the chamber with a cleaning rod. I also spent time at his house setting up his reloading equipment.

I am going to recommend some loads that have functioned my M1a's perfectly and functioned his AR10 perfectly. They are not manual max loads, but you will find, they are likely close to what you can push without malfunctions in your AR10.

A nice easy load, 150 grain bullet, 42.5 grains IMR 4895 LC cases, CCI #34. This should push a 150 grain bullet around 2700 fps in a 22 to 24 inch barrel. Is very close to the 30-06 ball load.

A target load, or a hunting load, 165 or 168 grain bullet, 40.5 grains IMR 4895 to 41.5 grs IMR 4895 LC cases, CCI #34. Don't think because you are using commercial cases that you can bump up this load without shooting a couple of hundred rounds. I got my bud to try 40.5 grains of his AA2520 with 165's and he has been killing them deer dead at 300 yards. Functions perfectly.

As for powders, your first choice should be any of the IMR 4895 series. That is IMR 4895, H4895 and AA2495. Start with a 165/168 and 40.5 grains and work your way half a grain at a time till you hit 41.5 grains. You may have extraction and function issues before you get to 41.5 grains, if so, cut back by 1/2 grain till pressure problems go away.


Only difference I can tell between IMR 4895 and H4895 is that one is green and the other is black.



Powders I have used and can recommend for the gas guns are IMR 3031, AA2520, IMR 4064. IMR 4064 is as slow as you should go and IMR 3031 is as fast. I don't recommend Varget as I have had issues, it is a bit slow. Great bolt gun powder, too slow for my gas guns.

Always full length size with these rifles, I prefer small base dies for function. Standard base dies may stick cases in your chamber. Set up your dies with case gages like these: I love this Sheridan gage, it is actually cut with a minimum SAAMI reamer unlike the Wilson gage.



This Wilson gage is cut large between shoulder and base, so you will get the cartridge headspace correct, but it won't tell you if your cases are fat. But, regardless of gage, use a gage to set up your dies. Size to gage minimum or, if you know the chamber headspace, bump the shoulder back 0.003"



Always prime by hand and ensure that primers are below the case head. I recommend CCI #34's or Tula Mil spec to reduce the chances of a slamfire. Unlike Garands/M1a's, the Stoner action does not slamfire out of battery, but they still slamfire. If you can't find the mil spec primers, use CCI standard. Don't use Federal as they are the most slamfiring primer on the market. Seat your bullets to less than 2.800", I seat my 150 to 2.750". Don't try any stupid ideas about seating to the lands. Keep those bullets off the lands.

[Unclenick]

Have you found the slotted gauges to be an advantage? I like that they are reamed gauges (big shortcoming of the other brands, IMHO), and I like that you get to see how much extra neck room there is through the slot, but at 40% higher cost I am wondering if you have found it has other advantages?

The jamming case story is useful information about that particular gun. I don't own one, so I was unaware of that, though I never load my M1A hot, either. A friend of mine built a 1000 yard Garand with heavy match barrel and a ported gas plug that we loaded to good effect with Varget under the 175 grain SMK. I'm not going to say how much, here, because it was well beyond the current published maximum and it was long enough ago that some lots of Varget were slower than others and perhaps slower than current Varget, but with the vented plug for the gas system it worked fine and showed no pressure signs.

[Slamfire]

Have you found the slotted gauges to be an advantage? I like that they are reamed gauges (big shortcoming of the other brands, IMHO), and I like that you get to see how much extra neck room there is through the slot, but at 40% higher cost I am wondering if you have found it has other advantages?

I agree the slotted gage is an expensive toy. Clearance forward of the neck was nice to see, did not know I had that much. I can see just a little clearance between the case and gage, that is reassuring. So far, none of my cases have been an interference fit, so the gage is not revealing any defects. I can't really justify the extra cost other than, I love useful gadgets, and this is one.

The jamming case story is useful information about that particular gun. I don't own one, so I was unaware of that, though I never load my M1A hot, either. A friend of mine built a 1000 yard Garand with heavy match barrel and a ported gas plug that we loaded to good effect with Varget under the 175 grain SMK. I'm not going to say how much, here, because it was well beyond the current published maximum and it was long enough ago that some lots of Varget were slower than others and perhaps slower than current Varget, but with the vented plug for the gas system it worked fine and showed no pressure signs

Different lots have different pressure curves as you are well aware. Based on my experience, Varget is on the slow side, if your rifle likes it, good for you. I prefer faster to slower in gas guns, faster powders mean less chamber pressure at unlock. High residual breech pressures at unlock are undesirable and will cause malfunctions. I was not using a ported or vented plug on my M1a. Some of these AR10 type actions do have adjustable gas systems, but when you get down to it, I will recommend powders that are appropriate for the gas system. I would not, nor recommend to new reloaders that they can adjust the gas system and then maybe use a powder that is inappropriate for the rifle. Might work well, might not. And since IMR 4895 was the powder that the Army used in the early development of the 7.62 cartridge, that is the best starting place for everyone.

This is my source for this claim:

History of T65 Cartridge

Reference: page 49 Oct 1973 American Rifleman

The preliminary drawing for the experimental cal 30 short cartridge was completed at Frankford Arsenal on Dec 12 1944, under the nomenclature Cartridge Ball, Cal. 30, T65. The T designation indicated a test items. The preliminary load development work was done at Aberdeen Proving Ground early in 1945.

On March 6, 1945, Frankford Arsenal was directed to produce 15,000 T65 cartridges for experimental test barrel firing. Manufacture of this ammunition, using IMR propellant, was completed in August 1945. This loading gave an average instrumental velocity of 2600 fps at 78 ft from the muzzle.

After the initial production by Frankford Arsenal, development of the T65 cartridge continued with the assistance of Olin Mathewson chemical Corp and Remington Arms Co. On Dec 15 1953, the final form of the T65 cartridge, the T65E3, was adopted as the 7.62 mm NATO by the North Atlantic Treaty Organization nations of Belgium, Canada, France, United Kingdom, and the United States. Formal standardization of this round as a U.S. rifle-caliber military cartridge occurred in August 1954.

National Match ammunition was loaded with IMR 4895, though of a lot and burn rate that is not available over the counter.