AR10 overpressure signs with factory ammo

[AggieEE]

Thanks Vzerone. I'll give it a try.

[308Jeff]

Subscribing to this to see how it works out.

[vzerone]

Thanks Vzerone. I'll give it a try.

I came up with that because of all the S&W's I have and they are 5 groove. That is until I bought a V mic. Hope it works for you and hope you understood my crummy instructions.

[AggieEE]

Quick update, I tried vzerone's bent angle. Seems to work but my difference between bullet dia and bullet and "V" changes with bullet dia. Which tells me there is a curve in the bends but beggars can't be choosy. Using a .308 bullet then checking my slug I get .308 for my barrel dia. so far so good. It was suggested that the firing pin was pulling out primers. I checked one of the cases that was missing a primer. I could drop a primer into the pocket so the pocket was expanded. I don't think pulling a primer under pressure would cause the primer pocket to expand, would it? I'll do a throat pound cast when I get a chance just to finish off all of the suggestions. I'm starting to feel that that ammo is just a little warm, ok hot, I still need to try it in my bolt gun just to double check. But, one gun's safe load is too hot in another so ???

[vzerone]

I wouldn't think the primer pocket would get expanded after the primer is out. What's happening is the pressure is very high, and being a cup, the primer seals off the back end of the case and the pressure pushes the walls of the primer not only against the bolt face, but against the walls of the primer pocket, thus the reason the primers pop out. Yes sounds like you have some very hot ammo. I hope you didn't shoot all of it or throw the box away because if you can prove it was hot you can contact the manufacturer of the ammo which in your case is Federal. First check that throat.

[tomme boy]

too much gas and the bolt is unlocking too early. Typical AR10. Very common problem. Get a adjustable gas block and be done with it.

[John 242]

too much gas and the bolt is unlocking too early. Typical AR10. Very common problem. Get a adjustable gas block and be done with it.

Does over-gassing cause primer pockets to expand to the point that primers fall out and new primers to drop into the pocket without resistance?
If so, would you please explain how that happens?

This from the OP-

I checked one of the cases that was missing a primer. I could drop a primer into the pocket so the pocket was expanded.

[vzerone]

Over gassing does not cause primer pockets to swell. Over gassing does cause the primer to severly flatten giving a false indication of an over pressure round.

[John 242]

Over gassing does not cause primer pockets to swell. Over gassing does cause the primer to severly flatten giving a false indication of an over pressure round.

Any other signs, like maybe peening to the corners of the locking lugs, maybe failures to extract and/or damage to the rims of the case, resulting from the case sticking in the chamber, caused by higher than normal pressures?

I seem to recall that higher pressures can cause all of the above.

[akajun]

Gas port has nothing to do with this, if anything an oversized port would theoretically reduce pressures.
Sounds like you either have a tight chamber , tight headspace, or short throat. Do you have a set of hs gauges?? See if it will accept a go.
Next use your stony point/ Hornady oal gauge and measure the throatvwithbthe bullet your using.
Last what chamber is on that barrel? I have a 308 match reamer from ptg that will show signs of over pressure with moderate loads. I now use either an obermyer or m118 reamer on my gas guns, both shoot the 150-175gr matchkings really well.
Btw may want to try some different ammo, including some 762x51 surplus. While 556 is higher pressure than 223, the reverse is true for 308 I’ve found. If your chamber is cut for 762 that might explain your issues.
Let me know what you find, we can get this worked out.

[vzerone]

Gas port has nothing to do with this, if anything an oversized port would theoretically reduce pressures.
Sounds like you either have a tight chamber , tight headspace, or short throat. Do you have a set of hs gauges?? See if it will accept a go.
Next use your stony point/ Hornady oal gauge and measure the throatvwithbthe bullet your using.
Last what chamber is on that barrel? I have a 308 match reamer from ptg that will show signs of over pressure with moderate loads. I now use either an obermyer or m118 reamer on my gas guns, both shoot the 150-175gr matchkings really well.
Btw may want to try some different ammo, including some 762x51 surplus. While 556 is higher pressure than 223, the reverse is true for 308 I’ve found. If your chamber is cut for 762 that might explain your issues.
Let me know what you find, we can get this worked out.

An oversized gas port does not reduce pressure. The larger the gas port the higher the pressure and the more volume of gas going back to the bolt/carrier group. With short barrels the gas port is brought more near the breech end of the barrel and it's smaller in size because the gas pressure is higher the closer to the breech you get. On some short barreled pistol AR's that are still using a DI system the gas port can be very near the chamber because a pistol cartridge doesn't have enough gas volume and pressure to operate the action.

[popper]

Does over-gassing cause primer pockets to expand to the point that primers fall out and new primers to drop into the pocket without resistance?

Not normal but can happen. Usually, PP is expanded as primer cup is expanded by high pressure. Ok. Now what happens if primer is pushed out a tad, then crammed back in? Kind of a 'top-hat' on the cup. Big reason H.S. and gas are important. Did OP ever check ammo with case gauge? And see if it 'fits' his rifle?

[vzerone]

Not normal but can happen. Usually, PP is expanded as primer cup is expanded by high pressure. Ok. Now what happens if primer is pushed out a tad, then crammed back in? Kind of a 'top-hat' on the cup. Big reason H.S. and gas are important. Did OP ever check ammo with case gauge? And see if it 'fits' his rifle?

Over gassing can never expand a primer pocket that the case is no good anymore. Only a very very high chamber pressure can do that. Please don't perpetuate a new myth. Over gassing will cause a harsh action cycling and also starts to open the action earlier then designed to. Just switch a swollen primer pocket over to a bolt action rifle that has no gas system. See my point?

[akajun]

I think you are confusing gas pressure from firing, which is what causes the powder to burn and create pressure to push the bullet down the bore , with increased gas flow/ volume from the gas port which is used to cycle the action. The pressure is the same from an oversized port or an undersized port, it’s the volume that’s different . If anything, bleeding off more gas rim an oversized port would bleed off pressure, but I believe that the powder has all burned up by the time the bullet passes the gas port on an ar10 anyway so peak pressure has already been achieved.
He has a chamber / throat/ hs issue , evidence is the popped and expanded primers, that’s handloafing 101 for signs of overpressure.
If he was ripping off rims or throwing cases 30 feet then I’d say he’s over gassed.

[vzerone]

I think you are confusing gas pressure from firing, which is what causes the powder to burn and create pressure to push the bullet down the bore , with increased gas flow/ volume from the gas port which is used to cycle the action. The pressure is the same from an oversized port or an undersized port, it’s the volume that’s different . If anything, bleeding off more gas rim an oversized port would bleed off pressure, but I believe that the powder has all burned up by the time the bullet passes the gas port on an ar10 anyway so peak pressure has already been achieved.
He has a chamber / throat/ hs issue , evidence is the popped and expanded primers, that’s handloafing 101 for signs of overpressure.
If he was ripping off rims or throwing cases 30 feet then I’d say he’s over gassed.

I'm not confusing anything sir. I know very well how the AR and it's DI system works. I believe I mentioned both volume and pressure. The pressure going back to the bolt/carrier group is quite high, more then many would believe. I'm trying to get across the point his problem isn't the gas system and the gas system has no correlation with swollen primer pockets.

The OP was suppose to fired the other ammo which I don't believe he has. If that shows high pressure signs it just about leaves out the ammo.

[popper]

Over gassing can never expand a primer pocket Correct, I stated that improperly. IMHO he has a H.S. problem (or bad ammo), the overgassing just doesn't help - he got ejector dents and bad primers too. He needs to get a case gauge and check HIS rifle's H.S. with the factory ammo. Overgassing just slams the case head into the bolt - not good. Once fired case normally doesn't show a 'ring' of weakened case head. Extreme pressure will extrude brass around the ejector and FP hole. If he just taps any 308 jacketed into the muzzle he can easily measure bore and groove. Loose barrel nut can be the problem also, this appears to be a self assembled rifle.

[vzerone]

Over gassing can never expand a primer pocket Correct, I stated that improperly. IMHO he has a H.S. problem (or bad ammo), the overgassing just doesn't help - he got ejector dents and bad primers too. He needs to get a case gauge and check HIS rifle's H.S. with the factory ammo. Overgassing just slams the case head into the bolt - not good. Once fired case normally doesn't show a 'ring' of weakened case head. Extreme pressure will extrude brass around the ejector and FP hole. If he just taps any 308 jacketed into the muzzle he can easily measure bore and groove. Loose barrel nut can be the problem also, this appears to be a self assembled rifle.

We're good popper. I like ya pardner! Yeah I think it was David Tubbs that came out with his carrier weight systems to delay that bolt opening action. The best way to have done it was with an adjustable gas valve because adding weight to the bolt/carrier group just slams the hell out of your barrel extension and the group itself.

He did a lead slug of the barrel popper. I drew him that V tool so he could mic it as the barrel is a five groove.

Aggie please do a test firing of the other ammo so we can see if it does the same thing with it.

[John 242]

Over gassing can never expand a primer pocket that the case is no good anymore.

This is what I was thinking as well and thank you for clearing that up.

Aggie please do a test firing of the other ammo so we can see if it does the same thing with it.

If I had the gun in front of me, I would first check headspace, just because, then I would test fire it with another brand of (good quality) ammo, as you suggest.

I don't "think" headspace has anything to do with this, but it makes sense to work analytically and scratch that off the list as well. And then ... ?

I have seem problems lately with guys firing Wolfe steel case .223s in several different AR-15 type guns. Cases stick in the chamber and the extractor overides the rim, tearing it up. They will also have flattened primers. The guns headspace fine and brass cased ammo extracts and ejects fine. I can speculate as to the cause of the problem, but simply changing ammo "fixes" it.

[6.5/06]

Forgive me if it has been mentioned(didn't see it), but the first handloads I fired in my AR 10 had similar problems. (even though several grs. below max charge). It was the new Fed brass I used, had soft case heads. Seems Fed has a reputation for this. They said to ship it back to th

[AggieEE]

Short update again. Took me a while to touch base with my gunsmith, work weather etc, and the bolt would lock up an a no-go gauge. Next step will be to talk to Brownell's about the barrel and see where it goes from there. I don't see how excessive head space would expand primer pockets but fix one problem and test again. I may have two issues at the same time or not. Will update as I learn things.