Is 6.5 Grendel SAAMI 52,000 PSI spec out of date?

[lazyengineer]

I can't help but wonder if this original spec still really applies to current AR's with current metallurgy and practices. I know early on people where having issues breaking bolts, and early on is when the SAAMI spec pressure was set. But today, I can't recall the last time I've seen a thread complaining about a broken bolt. And I see plenty of threads of people likely going above SAAMI pressure - at times, WAY above SAAMI pressure. Yet they seem to keep chugging along just fine and not breaking bolts. I personally have a couple pet loads that are giving me velocities that make me suspect I'm probably crowding it - and may indeed be a bit beyond. Yet seem to be having no issues.

I've tried using the internal ballistic calculator to estimate internal pressures and it says most loads (including published ones) are going WAY above SAAMI pressure, yet seem to be doing just fine. Separate topic I suppose, as I personally don't think Gordons (or Quickload) are very accurate for 6.5 Grendel; but even so, if they are saying I'm running well into high 60,000 PSI+, is it unreasonable to think that maybe I'm above 52,000 at least?

Kind of curious if this is something that should be revisited, as I'm beginning to suspect that 52,000 psi spec is a bit out of date and overly conservative.

Thoughts?

[BluntForceTrauma]

I wish we knew — and I don't know if Bill Alexander knows — what the average expected 65G bolt life is for 52K-max factory loads versus the average expected 556 bolt life with factory loads.

[Happy2Shoot]

the average expected 556 bolt life with factory loads.

When I was active duty I had an extractor wear out. I asked the armorer, while he swapped out extractors, how many rounds does it take to wear out an extractor and he said 10,000. So from that I assume they can take at least 10,000 M855 rounds. Just my two cents.

[BluntForceTrauma]

Did he say how many rounds to wear out the bolt itself?

[JASmith]

I can't help but wonder if this original spec still really applies to current AR's with current metallurgy and practices. I know early on people where having issues breaking bolts, and early on is when the SAAMI spec pressure was set. But today, I can't recall the last time I've seen a thread complaining about a broken bolt. And I see plenty of threads of people likely going above SAAMI pressure - at times, WAY above SAAMI pressure. Yet they seem to keep chugging along just fine and not breaking bolts. I personally have a couple pet loads that are giving me velocities that make me suspect I'm probably crowding it - and may indeed be a bit beyond. Yet seem to be having no issues.

I've tried using the internal ballistic calculator to estimate internal pressures and it says most loads (including published ones) are going WAY above SAAMI pressure, yet seem to be doing just fine. Separate topic I suppose, as I personally don't think Gordons (or Quickload) are very accurate for 6.5 Grendel; but even so, if they are saying I'm running well into high 60,000 PSI+, is it unreasonable to think that maybe I'm above 52,000 at least?

Kind of curious if this is something that should be revisited, as I'm beginning to suspect that 52,000 psi spec is a bit out of date and overly conservative.

Thoughts?

If the ballistics calculator disagrees with the published pressure by more than 1,000-2,000 psi in the Grendel, then the calculator is very likely to be wrong.

See http://shootersnotes.com/velocities-and-...-pressure/ for a discussion of the SAAMI Pressure Standard.

Determining whether the error cause is bad powder coefficients or user error is an exercise for the student.

[Happy2Shoot]

Did he say how many rounds to wear out the bolt itself?

Sorry, no.

I was a gun guy back then, but not an AR15 guy.

[Fess]

Looking around ARFCOM and M4.net I saw several references to a study that showed average bolt life was about 12,000 rounds for both M16's and M4's. Interestingly, a lot of bolt showed cracks at 6,000 rounds but did not lose a lug until much later.

[LRRPF52]

Bolt failures were often with after-market low-grade parts, like Model 1 Sales made with 8620 and improperly heat-treated, and they didn’t match Grendel bolts dimensionally or metallurgically.

I’ve been shooting high volume through 16” AA barrel/bolt, 17.6” Lilja/Maxim/PF bolt, and 12” Faxon Monster Group Buy/Monster Group Buy bolt, starting with the AA barrel/bolt in 2009.

Went to the Lilja in 2014 and shot it extensively from 2014-2019, almost my exclusive long range platform.

Started shooting the 12” Grendel in 2017, suppressed exclusively since Jul 2018.

I have moving box piles of Grendel brass from these and multiple other Grendels that I have built and accuracy-tested.

I have also had a lot of Grendels come through DM and LR courses from 2010-present, where we shoot a minimum of 225rds in 2 days every one of those courses.

I just haven’t seen a Grendel broken bolt yet, but I know there are people who have done it. I have broken 5.56 bolts that were run in guns through torturous high volume CQM sessions or training for 3-Gun. I broke my first one at over the 10,000rd mark in about a year. That was CLGS 16”.

For my 12” CLGS suppressed and now the 17.6” Lilja MLGS, I’m using Bootleg Adjustable Gas Carriers to mitigate excessive pressure into the Stoner Internal Expansion gas system.

[StoneHendge]

I broke my 2 bolts about a year ago. One was a JP.

[Bonas]

SAAMI uses a 24" barrel for Grendel. They state a max pressure velocity of 2,580 fps for a 123 grain bullet. The specs are for ammo manufacturers and are designed for reliability as well as safety, along with other factors. I get 2,570 fps from Hornady Black 123 HPBTs from an 18" barrel. It seems to me that at least one manufacturer is loading at least one load beyond 52,000 psi average pressures.

There's a thread in the reloading forum about pressure signs for Grendel. The consensus holds that SAAMI max pressure of 52,000 psi will be exceeded for Grendel before traditional pressure signs like flattened primers will appear. I'm sure that's true. However, it raises the question of whether safety or reliability is the main driver behind the SAAMI spec for Grendel. In other words, was 52,000 psi chosen primarily to protect bolt life or prevent catastrophic bolt failure?

The last SAAMI update came in 2015.

[Fess]

Bill Alexander had stated that the SAAMI pressure max was set to prevent bolt breakage in an AR15 with minimal modifications. Bolt backforce is what must be resisted, and the worst case of that is the "piston" of the area of the case head pressing backwards at full chamber pressure. The larger the case head, the more backforce. The case head of the Grendel is larger than that of the 5.56x45, so it has to operate at a lower pressure to prevent breakage.

With the 6.5 Grendel, the bolt and corresponding barrel extension from the AR15 family was not enlarged from its 5.56x45 predecessor. That was a deliberate choice to make the conversion to 6.5 Grendel as simple and inexpensive as possible. It is not that it could not be done, for example, Olympic Arms briefly made AR15's that shot the short, fat .223 and .243 OSSM cartridges (variants of the WSSM line). CMMG currently uses a larger bolt and barrel extension for its Grendel and 7.62x39 rifles and could likely handle more pressure as well.

Although it is possible to use exotic materials like maraging steel (as used in SIG's new 6.8x51 rifle) to make the current-sized bolt stronger, the 65 Grendel case itself can become the limiting factor as pressure is increased. The case was designed around the 52,000psi SAAMI pressure limit. The case walls near the base were kept relatively thin to maximize powder volume. This is generally not a problem for bolt action rifles since they often provide chamber support all the way to the case head. In autoloaders, however, the situation is different since they violently yank the case out before all the pressure in the barrel has subsided. The chambers also tend to be looser, especially near the entrance, to enhance feeding and reduce jamming problems.

[Bonas]

Bill Alexander had stated that the SAAMI pressure max was set to prevent bolt breakage in an AR15 with minimal modifications. Bolt backforce is what must be resisted, and the worst case of that is the "piston" of the area of the case head pressing backwards at full chamber pressure. The larger the case head, the more backforce. The case head of the Grendel is larger than that of the 5.56x45, so it has to operate at a lower pressure to prevent breakage.

With the 6.5 Grendel, the bolt and corresponding barrel extension from the AR15 family was not enlarged from its 5.56x45 predecessor. That was a deliberate choice to make the conversion to 6.5 Grendel as simple and inexpensive as possible. It is not that it could not be done, for example, Olympic Arms briefly made AR15's that shot the short, fat .223 and .243 OSSM cartridges (variants of the WSSM line) for example. CMMG currently uses a larger bolt and barrel extension for its Grendel and 7.62x39 rifles and could likely handle more pressure as well.

Although it is possible to use exotic materials like maraging steel (as used in SIG's new 6.8x51 rifle) to make the current-sized bolt stronger, the 65 Grendel case itself can become the limiting factor as pressure is increased. The case was designed around the 52,000psi SAAMI pressure limit. The case walls near the base were kept relatively thin to maximize powder volume. This is generally not a problem for bolt action rifles since they often provide chamber support all the way to the case head. In autoloaders, however, the situation is different since they violently yank the case out before all the pressure in the barrel has subsided. The chambers also tend to be looser, especially near the entrance, to enhance feeding and reduce jamming problems.

Right in line with what you're saying, SAAMI max average pressure for 6mm ARC is 52,000 psi. That spec is just over two years old.

[LR1955]

Bill Alexander had stated that the SAAMI pressure max was set to prevent bolt breakage in an AR15 with minimal modifications. Bolt backforce is what must be resisted, and the worst case of that is the "piston" of the area of the case head pressing backwards at full chamber pressure. The larger the case head, the more backforce. The case head of the Grendel is larger than that of the 5.56x45, so it has to operate at a lower pressure to prevent breakage.

With the 6.5 Grendel, the bolt and corresponding barrel extension from the AR15 family was not enlarged from its 5.56x45 predecessor. That was a deliberate choice to make the conversion to 6.5 Grendel as simple and inexpensive as possible. It is not that it could not be done, for example, Olympic Arms briefly made AR15's that shot the short, fat .223 and .243 OSSM cartridges (variants of the WSSM line) for example. CMMG currently uses a larger bolt and barrel extension for its Grendel and 7.62x39 rifles and could likely handle more pressure as well.

Although it is possible to use exotic materials like maraging steel (as used in SIG's new 6.8x51 rifle) to make the current-sized bolt stronger, the 65 Grendel case itself can become the limiting factor as pressure is increased. The case was designed around the 52,000psi SAAMI pressure limit. The case walls near the base were kept relatively thin to maximize powder volume. This is generally not a problem for bolt action rifles since they often provide chamber support all the way to the case head. In autoloaders, however, the situation is different since they violently yank the case out before all the pressure in the barrel has subsided. The chambers also tend to be looser, especially near the entrance, to enhance feeding and reduce jamming problems.

Fess:

Thanks for the info. Folks need to remember the intent behind the Grendel which was more tactical than target.

When Oly Arms was still operational I lived close by and they gave me a converted 5.56 bolt that was actually pretty durable. Better than Colt 7.62 X 39 bolts (I broke two of them), and lasted as long as one of the AA / CSS 7.62 X 39 bolts and one AA Grendel bolt. Yes, they were making some really fat and short cartridges that actually functioned through the AR. I currently have a M-4 they made that I have shot a gigantic amount of service ball and handloads out of with absolutely no issues.

LR-55

[LRRPF52]

I’ve had this conversation with Bill many times, and there were many considerations that went into specifying the Maximum Average Pressure.

He could have engineered a rifle that would shoot the 6.5 Grendel cartridge at much higher pressures like a .260 Rem or 6.5x47 Lapua, but then you’re back into the recoil/blast problem from a lightweight AR-15 chassis.

This was something meant to be easy to shoot by hunters and target-shooters alike, where you don’t come off target so violently like with .308-based cartridges.

Engineering the action to take higher pressure would have required longer barrel extension lugs, longer bolt lugs, and a resultant longer-travelling bolt carrier in the action.

This would require a longer action and then you’re into an AR-10 at that point, which defeats the benefits of the lightweight, compact AR-15.

You could use exotic alloys for the bolt and extension if you wanted to toss all the recoil and muzzle blast penalties aside as acceptable compromises, but bolts and barrels would then cost significantly more. Figure a doubling of the cost for each bolt and extension into the equation. Bill’s forecast for the market was that the high pressure approach would reduce acceptance as to make 6.5 Grendel a very limited, low-exposure proposition and not worth the effort.

He also wanted an affordable drop-in upper receiver for those with existing rifles to capitalize on the AR-15’s modularity.