I had a partial case head separation using a 1.8oz buffer

[JASmith]

And another post directly addressing Lapua brass and how it articulates with the chamber under initial peak pressure, as well as during extraction:

Note that Bill A. said that the correct cone geometry will allow pressures in excess of 68 ksi. This happens to be in the range of published yield points for cartridges brass in tension.

Pressures higher than the yield point causes brass to flow. Significantly higher and the flow is fast enough that a hole opens in the unsupported part of the brass. That allows a jet of hot gas to fly through the action. This is not good for the action and, occasionally, the shooter.

Brass moving at lower pressures, e. g. a few thousand psi, particularly when the case obdurates the chamber, is almost entirely elastic. The elastic behavior lets the case spring back to allow extraction. There is, of course some plastic movement because the necessary clearances allow that much movement., which is why we need to size cases.

That tiny bit of plasticity also lets the case shoulder to move forward resulting in stretching. That is one of the reasons we pay attention to headspace. Avoiding bumping the shoulder unless needed to allow bolt closure increases the number of firings before the case separates at the base.

(Note that premature bolt opening n ARs gives similar symptoms but with a single shot.)

Also, aggressively squeezing the brass neck and shoulder region work hardens the brass. This results in longitudinal splits through the neck and, sometimes, the shoulder.

[BluntForceTrauma]

*edit - found it

https://www.65grendel.com/forum/showthre...post291053

Zen, good find. What I'm getting from that thread is the sizing die seems to be the common denominator, as in the YouTube video on the 6ARC "belted magnum" from Johnny's Reloading where you can see him form them in real time. Unless I skimmed that thread too fast, guys had fired the cases, but only discovered the belts after resizing them. Some assumed they occured on firing, but careful reading seems to point to the dies — at least in that thread.

[Zeneffect]

I was in it for the pictures of the cut case. Per the thread it seems these 2 events are entirely unrelated

[BluntForceTrauma]

Gotcha. Just points to the complexity of the issue.

[lazyengineer]

Zen, good find. What I'm getting from that thread is the sizing die seems to be the common denominator, as in the YouTube video on the 6ARC "belted magnum" from Johnny's Reloading where you can see him form them in real time. Unless I skimmed that thread too fast, guys had fired the cases, but only discovered the belts after resizing them. Some assumed they occured on firing, but careful reading seems to point to the dies — at least in that thread.

OP's Kaboom is raw unfired untouched brass and has some cases form belts.

[LRRPF52]

Some recent testing and pics. I have another pistol lower with a regular carbine tube and buffer (2.9oz). I took the Odin 12" out with the Odin bolt (I re-measured headspace and it passed fine - no idea what was going on earlier) and shot some Hornady Black factory loads (same bullet) and then a couple of my A2520 reloads. One of the 2 A2520 handloads had the ring, but a few of the Hornady cases also had a less pronounced ring starting, so I'm starting to think that I need an even heavier buffer (H3) on this CLGS setup.

I also pulled the remaining 9 that were left in the kaboom box, and segmented a Hornady case, and the 40th pre-kaboom case that had a ring. There is definitely a ridge inside the Lapua case.

The first pic is left to right: Lapua A2520 shot in carbine lower, Hornady black shot in carbine lower, and Lapua A2520 shot in pistol lower (1.8oz buffer)

The second and third pics show the Hornady black case from carbine lower and Lapua case from pistol lower. I didn't segment the Lapua case from the 2.9oz buffer (yet). Interesting that the outside rings are forming in different places (just above where the web ends, I bet). I have an adjustable buffer that I'll set up this week and do some more testing. Next up are heavier buffer, and then a different bolt (monster).

[ATTACH=CONFIG]19177[/ATTACH]
[ATTACH=CONFIG]19178[/ATTACH]
[ATTACH=CONFIG]19179[/ATTACH]

So the Hornady factory load’s brass that's sectioned, after being fired through the 12” upper with a standard carbine buffer, RET, and spring, still shows signs inside of the case down above the web of a failure node?

What’s the gas port diameter on the 12” barrel?

If your feed cone is within spec and is supporting the cartridge, you need to get a Bootleg Adjustable Gas carrier probably, or another type of carrier with adjustable gas.

I like the Bootleg the best because I just dial the gas adjustment through the ejection port without having to disassemble anything.

I run the 12” Grendels on the fully-choked gas setting since I shoot suppressed. I have enjoyed literally 4 years of reliability with it suppressed, maybe one quick cleaning of the BCG and chamber.

I’m using an extra power carbine-length recoil spring from Franklin Armory. You could also use a Geissele 42 coil woven spring.

I’m using a standard carbine buffer, 2.9oz/82grams.

The other 12” I built is using a Maxim PDW short RET with shorter, stiff action spring, Bootleg carrier, same barrel.

These barrels are 12” Faxon pipes spec’d by BluntForceTrauma for the group buy we did, with correct gas ports and feed cones.

I just checked a bunch of brass I have shot through it and do not see any unusual signs.

The other 12” has had 4 years of hand loads run through it using Lapua and Hornady brass shooting 107gr SMK mostly.

Both have been run suppressed the whole time with high efficiency TBAC Ultra suppressors, which tend to be harder on the cyclic rate if the action isn’t tamed.

The Maxim PDW RET, spring, and tiny buffer (also 2.9oz) actually runs smoother than the standard carbine RET with extra power spring and 2.9oz buffer, from my shooting perspective.

That little Maxim PDW set-up was one of the smoothest-shooting ARs I have felt, which was counter-intuitive due to 12” suppressed CLGS.

[Growler]

This is a great thread.

It is interesting that the OP and Johnny’s Reloading Bench both used Odin Works barrels. Given Bill Alexander’s observations that case support is a critical factor, it seems this may have uncovered two examples (despite different conditions) where less than ideal geometry led to negative results.

It would be interesting to measure Cerrosafe castings of the OP’s chamber and that of a known good one to see just how different the two case head support cones are. Perhaps the factor of safety concerns should center on case support rather than brass geometry?

Regardless, the early unlock/extract stresses seem to be the easiest to correct via mass regulation, gas control, and spring force as mentioned above.

[stonehog]

So the Hornady factory load’s brass that's sectioned, after being fired through the 12” upper with a standard carbine buffer, RET, and spring, still shows signs inside of the case down above the web of a failure node?

What’s the gas port diameter on the 12” barrel?

If your feed cone is within spec and is supporting the cartridge, you need to get a Bootleg Adjustable Gas carrier probably, or another type of carrier with adjustable gas.

I like the Bootleg the best because I just dial the gas adjustment through the ejection port without having to disassemble anything.

I run the 12” Grendels on the fully-choked gas setting since I shoot suppressed. I have enjoyed literally 4 years of reliability with it suppressed, maybe one quick cleaning of the BCG and chamber.

I’m using an extra power carbine-length recoil spring from Franklin Armory. You could also use a Geissele 42 coil woven spring.

I’m using a standard carbine buffer, 2.9oz/82grams.

The other 12” I built is using a Maxim PDW short RET with shorter, stiff action spring, Bootleg carrier, same barrel.

These barrels are 12” Faxon pipes spec’d by BluntForceTrauma for the group buy we did, with correct gas ports and feed cones.

I just checked a bunch of brass I have shot through it and do not see any unusual signs.

The other 12” has had 4 years of hand loads run through it using Lapua and Hornady brass shooting 107gr SMK mostly.

Both have been run suppressed the whole time with high efficiency TBAC Ultra suppressors, which tend to be harder on the cyclic rate if the action isn’t tamed.

The Maxim PDW RET, spring, and tiny buffer (also 2.9oz) actually runs smoother than the standard carbine RET with extra power spring and 2.9oz buffer, from my shooting perspective.

That little Maxim PDW set-up was one of the smoothest-shooting ARs I have felt, which was counter-intuitive due to 12” suppressed CLGS.

I don't quite want to give up on this barrel yet as it's light and a great shooter - easily the most accurate grendel I've had yet. Hoping I can tune this with the block and heavier buffer. The block is a bit coarse. Full cycle and lock back at 2 turns open - when I backed it off a quarter turn, no lock back. I may try 1/8th but it might be time for a superlative block or something nice. If/when I go that route, I'll measure the port.

[LRRPF52]

You can combine the Bootleg Adjustable Carrier with an adjustable gas block.

Dial the gas block down to 75% choke, then use the Bootleg to choke it again at the carrier.

It has 4 positions from fully open down to fully-suppressed.

[stonehog]

Interesting test session today - all body parts still intact - no further kabooms.

I put the new RET and heavy H3 buffer on the lower then took her out to the range for testing. First up, I closed all gas, and got no rings as expected - both Hornady Black and my Lapua brass looked great. I gradually opened up the gas by quarter turns and got to cycling and lock open at 2 full turns - just like the last several times. Brass looked good with no ring bumps. I shot a few more and everything seemed good - minor marks on the brass, but nothing that looked like the ringing I was seeing before. The new buffer and carbine RET/spring seems to have slowed the cycling rate enough to make this problem go away.

For giggles, I put a Monster bolt in the gun and sent some rounds. The brass looked even better with this bolt, but I developed a different problem. I had put the lock ring back in the gas system, and went to shoot a few more rounds, and started to have problems with cycling and extraction. The round stayed chambered and I had to hammer the charging handle to extract the round. This was consistent for the next 4 rounds. Any ideas around this one?

[Zeneffect]

Chamber a round, then eject it without firing. Still getting stuck? Headspace or dirty gun is all I can think of.

[stonehog]

Yeah - that was just a loose gas block. Fixed, opened a bit, and then tested 10-15 more rounds with no rings or issues. Looks like it was indeed the early cycling due to a too light buffer. I’m going to call this one solved.

[Klem]

Stone,

Jammed cases when you shut the gas completely off is normal in an AR. You need at least some gas to free that initial jam. This is why in Post#297 I recommended shutting it off almost completely (but not completely). Then shooting and opening the gas up slowly with 1-round mags until it locks back reliably on last round - but go no further.

As it is looking like aggravated early unlocking then your gas block should be enough with a heavier buffer. Spending more money on after-market springs and adjustable carriers will also work, but the cheapest option is to use what you already have and swap the buffer for a heavier one.

If the mouth of the chamber is too wide to be supportive then that is an issue that only a new barrel from a different manufacturer will fix. Fixing the early unlocking might solve the magnum belting and catastrophic ruptures, but it my still be on the edge of brass longevity. If the brass is now OK however, and brass longevity is within you definition of reasonable then problemo solvered.

[Rivercityjeff]

Amen Klem!

[stonehog]

More testing today - different powder (SW Match Rifle) but same 123 ELD-M bullets. I did two 5-round group/es tests and lo and behold I got a ringed Starline (new) on one of the cases in the second groups. At that point, I lost trust on this barrel. I'm going to contact Odin Works and see if I can send it to them for evaluation. BTW, when the gas block was off, I measured the gas port at around .076-.080? Hard to get an accurate measurement with my old RCBS caliper. Again - I closed the gas on this barrel and slowly (? turn) opened until I got a lock on last round. I feel like the gas is controlled, and this was with the Monster bolt and H3 buffer. Fun stuff. I think I'm going to order a 16" monster barrel in the mean time...

[VASCAR2]

The info I have shows Faxon Group Buy Monster barrel use .068” gas port with carbine length gas on 12” barrel. I think my 12” Green Mountain Monster barrel has .068” gas port and it functions great with suppled carbine buffer and spring. I’ve never used a suppressor as they are unlawful in Illinois.

[stonehog]

More pics. First one is a lineup of a Hornady Black factory case that had strange bulges - 2 lumps and a long vertical ridge. Like something hard was inside with the powder?. Then it goes left to right: Hornady, Lapua, Starline. Failed case in the back for reference. You can see the inside crack starting in Lapua and Starline, but there is also the beginnings of this in the Hornady case, too.

[ATTACH=CONFIG]19197[/ATTACH]

Here's a closeup of the sectioned Starline case:

[ATTACH=CONFIG]19198[/ATTACH]

And here is the exterior showing the rings. Left to right: Hornady, Lapua, Starline:

[ATTACH=CONFIG]19199[/ATTACH]

Odin got back to me on the barrel so I'm going to arrange to send it in for inspection.

[BluntForceTrauma]

Thanks for taking the time to report — learning experience for us all! Takes some effort to section all those cases.

[stonehog]

I'm getting better at it - the dremel is the tool

[grayfox]

I'm with you. A barrel I have no confidence in, I get rid of.
You need something you can feel confident about.
Barrels are cheap, eyeballs are not!