Severe Overpressure in a 300 BLK

[Boolseye]

Hey Folks,
Dad handed off his AR to me the other day, action stuck closed after a round had been fired.
With a steel rod, a brass drift and some elbow grease I managed to extract this:

We're thinking a bullet telescoped into the case, there had been some neck tension issues, he said. No damage to man or gun. It looks like about 80,000 psi to me–brass has flowed into a ridge near the case head where it started to extract and locked up. I'm wondering if any of your experienced eyes can suss this one out.

[BK7saum]

Naw, that's the new whizzbang rimmed 300 blackout belted magnum.

[BK7saum]

Seriously though what was the load and powder. If using AA1680, I'm setting on an almost full case already.

[44man]

Double charge of H110? Or bullet moved from the primer before ignition.

[dverna]

A double charge, if it fits in the case, should have caused more damage.

If the first shot of recent reloads, maybe the wrong powder, or the wrong charge. I would break down the rest of the loads and check them out.

Don Verna

[popper]

Show a pic of the case head. Bolt bounce? Jammed the lands or neck too tight? Could be over pressure causing early bolt unlock. When I get setback it usually is from hitting the ramp or lower with a blunt nose.

[Boolseye]

Thanks all, more pics coming. 44 man, that sounds plausible and fits with the deformation of the case.

[Mica_Hiebert]

Ive had the same issue with reman 5.56, dont know the cause it all went back to the manufacturer after 3 blown primers and a head separation in 1 magazine. An easier way to clear a round like that in an ar15 is to hold the bolt catch and pull on the charge rod while driving the butt of the rifle onto the ground.

[Tackleberry41]

I really doubt you could double charge w H110, that powder doesnt like to be downloaded more than a few percent, double would have over flowed and been everywhere.

But bullet set back is a risk with anything self loading. And many of the bullets being used in the 300 dont come with a cannelure where you can give them a proper crimp. The guy I know running a 300 is using the 208gr Amax, and has no real way to give a firm crimp. The Lee factory crimp might work, but know he doesnt have one. He has blown primers out in use. He blew off any advice I gave.

And he has 2 friends who went the opposite way and got 2 of those 208gr bullets stuck in a bore.

I recently bought a cannelure tool for something else but would probably work well w the 300.

[runfiverun]

without any load details we are just guessing at the pressure.
I know it isn't a double charge of h-110 or 1680 you'd never be able to seat the bullet with all the powder spilling on the bench.

I have had some neck clearance issues in my AAC.
but it has a tight chamber geared towards shooting sub sonic jacketed rounds so that's to be expected.

[SP5315]

I've seen that happen to my buddy. Case length was off and too thick of a case neck. Pulled his bullets, trimmed to the proper length, and sorted out cases that had to case necks too thick. The cases that were giving him problems were PPU and PMC.

[NoAngel]

I'd be interested to know as well. I had a bolt action 300BLK built on a 110 and have ran more Lil'Gun that any sane person ought to. Until I started really compressing it, I saw no pressure signs other than flat primers. I didn't see case flow until the charge was up to near the top of the neck.

That was stupid by the way, but I can't see Lil'gun giving that result either. Could it have been a fast pistol powder? Me thinks so.

[rda72927]

Wonder if the brass is factory 300 BKO or a converted 5.56? If converted 5.56, case neck thickness could be a cause.

[Boolseye]

It was a factory case. Probably used H110, still waiting on details. Will appraise soon as I get that info, thanks guys.

[yondering]

Lots of speculation in this thread.

The only things we can really say at this point is there was obviously a high pressure issue, and there could be a (lack of) annealing issue with that cracked case, meaning there may be more than one thing wrong at the same time.

I suggest we hold off trying to suggest answers until we know what the problem is.

OP, please provide details of the load, and the conditions in which this event happened.

[Boolseye]

Still waiting on details, guys, give me a minute....it may not be forthcoming. Here is the most likely load: 125 gr. Jacketed, full charge of H110. As mentioned, some loose bullets in other cases that he'd noticed, which is why I thought 44man was on to something.

[popper]

Stretched case, pressure ring on case head, stuck in chamber - bolt definitely unlocked early in firing cycle - or wasn't fully in lock. Pistol gas system? Lugs should keep the bolt locked until pressure is way down - that didn't happen? Maybe shoulder was just a tad long. Over crimp can also jam the neck enough. COAL usually isn't problem and factory case should be right dia. for jacketed unless an oversized spud (or small dia. bullets) used. Compressed lads of 110 work but can force the bullet up after seating. Check the FP for scraping damage. A 125 max load should give 40K pressure & no setback. A weird thought, check the gas key for clogging, it can keep the bolt from complete lock.

[Boolseye]

Factory case, Nosler 125 grain ballistic tip, 18 grains 4227. Carbine gas system.

[runfiverun]

well that combination would leave room for a bullet set back.

[chuckbuster]

Ultimate Cannelure Tool will help fix all your Bullet set Back issues.

http://castboolits.gunloads.com/forumdisplay.php?206-Chuckbuster

Kevin

[Moonie]

This is why I like the Lee FCD for the 300 BO.

[Boolseye]

Thanks guys.
-JP

[Hamish]

Boolseye,

I was loading a few to hunt with in the CVA pistol a few days ago and noticed uneven neck tension when seating 130 jacketed. I was being a little sloppy, these were in cases made by someone else. My fix has always been to run back through the FL die without the deprime/neck size stem and then seat without using a seater spud if possible and then hit it with the Lee FCD if I have one for that cartridge. This goes for both jacketed and cast IF the alloy can handle it with out being sized down by the neck.

I would NEVER run cases with loose neck tension in anything but a single shot and slow powder. Too scared to,,,,,,.

[yondering]

If there were some loose bullets, that brass either was not sized correctly, or has been worked too many times without annealing. You should not have to rely on crimp to avoid bullet set back in a 300 Blk AR, that's bunk. Make sure the cases have good neck tension, and go from there.

[yondering]

Stretched case, pressure ring on case head, stuck in chamber - bolt definitely unlocked early in firing cycle - or wasn't fully in lock. Pistol gas system? Lugs should keep the bolt locked until pressure is way down - that didn't happen? Maybe shoulder was just a tad long. Over crimp can also jam the neck enough. COAL usually isn't problem and factory case should be right dia. for jacketed unless an oversized spud (or small dia. bullets) used. Compressed lads of 110 work but can force the bullet up after seating. Check the FP for scraping damage. A 125 max load should give 40K pressure & no setback. A weird thought, check the gas key for clogging, it can keep the bolt from complete lock.

popper, supersonic 300 Blk runs at the same max pressure as the 5.56; they use the same cases and rifles, after all. That's a whole lot higher than "40K", unless you're using some weird units that nobody else does.

[pkie44]

Boolseye,


I would NEVER run cases with loose neck tension in anything but a single shot and slow powder. Too scared to,,,,,,.


Good advice.

[sfcairborne]

I know for my R700 blkout, I have to turn the neck on convert 223 brass and use the Lee crimp die. Zero problems now.

[Boolseye]

Just read the article on bullet set-out in the cannelure tool literature.
Apparently it's a thing with this round and this bullet.

[popper]

Uh, no. 17.7C from Hodgdon, max load 4227, 125gr Nos BT. No setback. 50K CUP, 40K psi is Sierra data for 1680. Been loading AR-BO for several yrs now. 125gr won't jam the lands or the bore, unless the primer did it. With that slower powder and high pressure/carbine gas may let the bolt open early. Early unlock is what happened, cause is unknown. Case pressure ~50K and gas tube pressure ~ 15K, guess which wins. Pistol gas would have been worse.
You still need to look at the crimp, it can buckle the neck and prevent complete bolt closure. I mic my necks after loading and if over 0.329 I pull them.
I use non-cannular bullets in the AR10 - I don't crimp into the cannular on any rifle, even 30/30. None of my moulds have a crimp groove.
Seriously, look at the gas key. I swapped BCG and could only get 20% bolt lock with an empty chamber AND the FP still worked.

[dtknowles]

I'd be interested to know as well. I had a bolt action 300BLK built on a 110 and have ran more Lil'Gun that any sane person ought to. Until I started really compressing it, I saw no pressure signs other than flat primers. I didn't see case flow until the charge was up to near the top of the neck.

That was stupid by the way, but I can't see Lil'gun giving that result either. Could it have been a fast pistol powder? Me thinks so.

I wonder how many other loaders here keep increasing the powder charge after they see flattened primers :)

[Soundguy]

I really doubt you could double charge w H110, that powder doesnt like to be downloaded more than a few percent, double would have over flowed and been everywhere.

But bullet set back is a risk with anything self loading. And many of the bullets being used in the 300 dont come with a cannelure where you can give them a proper crimp. The guy I know running a 300 is using the 208gr Amax, and has no real way to give a firm crimp. The Lee factory crimp might work, but know he doesnt have one. He has blown primers out in use. He blew off any advice I gave.

And he has 2 friends who went the opposite way and got 2 of those 208gr bullets stuck in a bore.

I recently bought a cannelure tool for something else but would probably work well w the 300.

10$ lee fcd die.. If using non cannelured bullets, it would be a no brainer.

Loved the 300 acc belted magnum comment :)

[yondering]

Uh, no. 17.7C from Hodgdon, max load 4227, 125gr Nos BT. No setback. 50K CUP, 40K psi is Sierra data for 1680. Been loading AR-BO for several yrs now. 125gr won't jam the lands or the bore, unless the primer did it. With that slower powder and high pressure/carbine gas may let the bolt open early. Early unlock is what happened, cause is unknown. Case pressure ~50K and gas tube pressure ~ 15K, guess which wins. Pistol gas would have been worse.


Popper, 40K psi is not max pressure for the 300, and 50K is not anything special or even a hot load. Go look at SAAMI specs, not just data for one load, 55,000 psi is SAAMI max pressure. You're inferring way too much from different gas system lengths too; the gas port size is far more important. The OP does look like it was an early unlock situation, but you're assuming way to much.

[Boolseye]

I suspect the problem was probably no or not enough crimp, and he had a bullet set-out situation, hence the early unlock and resultant case failure. He has since remedied this with the Lee FCD. This seems like the probable scenario in a perfectly functioning gun with a standard load.

[popper]

17.7 Compressed load from Hodgdon, max load 4227, 125gr Nos BT Even given some variance for the case it's still a compressed load. FCD isn't going to fix that. Gas length makes a difference in unlock time. I quit.

[Boolseye]

Thanks for your input, Popper, have a good Thanksgiving.

[yondering]

I suspect the problem was probably no or not enough crimp, and he had a bullet set-out situation, hence the early unlock and resultant case failure. He has since remedied this with the Lee FCD. This seems like the probable scenario in a perfectly functioning gun with a standard load.

Boolseye, FCD or any other crimp is not a cure for bad/inconsistent neck tension, it is only a band-aid fix. If you got bullet set-back (not set-out, not sure where that came from?), the case neck tension is probably inadequate; you really want to figure out the cause of that before the same issue happens again. The crack in that brass case is a good indication that the cases have been worked too many times and need to be annealed. The sizing die button could be expanding the necks too much as well, you can measure a sized empty case neck (inside) and compare to bullet diameter.

However, Popper's assertion that the compressed load would prevent set-back is correct, and you may need to look elsewhere for the answer. It's hard to know what else it could be though without a lot more detail.

[yondering]

Even given some variance for the case it's still a compressed load. FCD isn't going to fix that. Gas length makes a difference in unlock time. I quit.

Popper, gas length makes a difference, but port size matters a lot more; you're only considering one variable in a complex system. Either length can be severely over gassed with the wrong port size. There is nothing at all wrong with pistol length gas systems if the port size is correct, and that length is much more preferable for running subsonics and supersonics in the same setup. No need to get in a huff about it, just recognize that you might still have more to learn.

[tomme boy]

Bolt was not 100% locked. SOME AR's will still fire with about 50% engagement of the lugs. And if the PSI is too high like Popper is telling you WILL cause exactly what you are showing.

Lack of maintenance on the cleaning part will cause the bolt to not lock closed. I have taken a friends AR apart because he was having issues with inconsistent firing. Sometimes it would and other times it would not. His idea of cleaning was to oil the heck out of it and run one of them bore snakes through it. It took me almost an hour and a half to get all of the sludge and powder contaminants out of the bolt, carrier and upper rec. I found a couple of blown primers on the bottom of the lower. He tells me that it just does that every now and then.

Guess what? It does not happen at all since I cleaned the gun and chewed his butt for what he was doing. So don't over look a simle maintenance full cleaning of the gun.

[tomme boy]

And yes port size does make a difference too. Most manufacturers make them too large to make sure extraction will work. Thats why 90% of all carbines throw the brass at about 1 o'clock. Over gassed.

[Boolseye]

I appreciate all the feedback. I recognize you're all trying to help prevent an accident, and I'm grateful. Thanks again, all points are being considered.

[yondering]

And yes port size does make a difference too. Most manufacturers make them too large to make sure extraction will work. Thats why 90% of all carbines throw the brass at about 1 o'clock. Over gassed.

Yes, that is very true, for both 300 and 5.56 AR barrels these days.

I agree the bolt was probably not locked back. If the gun is oiled and not filthy dirty, it could be caused by an ammo problem. Over-crimp, as already mentioned, or just insufficient sizing.

It might be a good idea to run the rest of those rounds (if there are any left) through the gun manually, and make sure each one chambers fully and extracts easy. You might only have a couple in that batch that do it, but one is more than enough. :)

[Boolseye]

Thanks Yondering, he cycled the rest of his ammo without incident.

I built this AR for dad, has a Wilson Barrel and has never had an issue before or since this incident. We're calling it an ammo problem-I'll revive the thread if any further problems persist. In the meantime, safe shooting guys, have a good holiday.