Crazy Chamber Pressures in M855A1

[Bartholomew Roberts]

Lately, the Army has adopted the new "green" M855A1 round to replace the M855 round. One of the features of this round is an increased muzzle velocity (about 100fps).Several online sources that I've followed for around 10 years now are reporting M855A1 chamber pressures in excess of 62,000 PSI.

The original M855 spec was that chamber pressure should be 55k PSI, and that the mean chamber pressure plus 3 standard deviations should not exceed 61K PSI.

So this prompts a couple of thoughts:

1. Higher muzzle velocity + better BC in M855A1 means that the new round isn't going to track and of the existing iron sights and optics in terms of drop. I believe the Army has already encountered and identified this problem.

2. Originally, I figured they were using something like Hornady's new Superperformance to get the speed boost; but if those pressures are correct, it looks like they are just ramping up pressure dangerously.

3. Higher chamber pressure is bound to affect port pressure as well which is ultimately going to effect reliability during extraction and ejection.

4. Higher pressure is going to make existing wear issues even worse, which is going to multiply the reliability problems mentioned above.

5. Seems to me like between bullet setback, oil in the chamber or any other issue that would normally cause a minor pressure increase could result in a much more serious problem with M855A1 if theses pressures are correct.

Is anyone out there using M855A1 seeing any signs of high/dangerous pressures on ejected brass? Normally, I would tend to discount this as Internet nonsense; but the people reporting this issue includes some pretty well-known experts in the field of small arms development.

 

[Loader9]

Since you haven't had any responses to this post, I'll give you a heads up. The info you have posted is wrong. You're confusing SAAMI standards and testing to that of military- sorry, can't do that. It's like comparing CUPs and PSI, yeah the cartridge pressures are the same but the test methodology is different, hence, different results. SAAMI and military tests don't use the same transducers and they don't test at the same location on the brass. The military tests at the neck where pressures are highest and SAAMI tests just in front of the head. The correct pressure for military ammo is 340.00 MPa which equates to 62366 PSI. The allowables are 10% but they usually will trim the powder load to compensate over the average spread. So pulling a bullet to see how much powder the military used and then attempting to duplicate using the same military powder but of a different run number can bring disastrous results. The military powders are not required to have such close burn rates tolerances as the canister stuff we buy by the pound.

 

[Bartholomew Roberts]

You're confusing SAAMI standards and testing to that of military- sorry, can't do that.

I'm aware of the difference, and I'm assuming that the people who are reporting this are aware of it as well since they work in the small arms industry and since they cited the mil-spec for measuring M855 pressure. In addition, one reported obvious pressure signs on the brass, which wouldn't be caused by a measuring difference.

You can follow the link above to the original source if you still think that confusion between SAAMI and Military standards is the issue; but I don't think that is the case.