AR10 Pressure Issues w/ Factory Ammo

[squirrelsnpr]

I recently traded for a custom AR10. It shoots well, but I'm having some issues with it. Shooting Hornday Custom ammo with 155 A-Maxs and with TAP 168 A-Maxs, and with USA ammo 155 A-Maxs, I have blown primers. I have noticed the bullet appears to be coming into contact with the side of the chamber. I have ran some SouthWest 175 SMKs through it, and it shot ok. I have two FTF. I don't know if the chamber wasn't thoated for the A-Maxs, or what the deal is.

Along with blowing primers, I have had cases stick in the chamber (FTE). The part that is really scary is that the extractor ripped off part of the case rim it "grabs" and left the case stuck in the chamber. I had to use a cleaning rod to push the case out of the chamber.

I'm not a gunsmith or too familar with the mechanics of the AR platform, but here is what I'm thinking; the chamber was not cut square to the receiver which is causing excessive pressure in the chamber (over expanding the brass and popping primers). The excessive pressure is then getting into the gas tube and over pressurizing the bolt carrier. This could be causing the the bolt carrier to operate pre-maturely, or with too much force which is why the case rims are getting pulled off.

What are your thoughts?

 

[jakhamr81]

Re: AR10 Pressure Issues w/ Factory Ammo

It would help to know the manufacture of the rifle/ barrel. I would assume Armalite, but many fellas on here refer to every AR-308 as an AR-10.

<span style="font-style: italic">Edit</span> Just saw that you mentioned it is a custom. What receiver is it on? Try to find out who's barrel it is and what reamer was used to cut the chamber. Was the gun built by a gunsmith or assembled in someone's garage?

 

[BCP]

Re: AR10 Pressure Issues w/ Factory Ammo

Could be improper headspace, could be gas port size is too large, could be both. What do your fired cases measure like in relation to unfired brass?

edit: where are you in CA?

 

[Badshot308]

Re: AR10 Pressure Issues w/ Factory Ammo

Could be incorrect buffer, or buffer spring or both. Clint Bulter, known as "Slash" at AR15.com, specializes in these products http://www.heavybuffers.com/ What components were used to build this custom AR10?

 

[squirrelsnpr]

Re: AR10 Pressure Issues w/ Factory Ammo

I don't want drop the names of the barrel manufacture and custom builder because they are working with me on this issue.

The headspace was GTG. I am unsure of the gas port size. At this time, I do not have any unfired brass. Would the measurements of the fired brass help?

 

[BCP]

Re: AR10 Pressure Issues w/ Factory Ammo

Maybe if you were one of those "make a hot load and re-load it 20X times" I could understand a ripped case rim but factory ammo sticking in the chamber like a sonofabitch doesn't sound like good headspace to me. Take it to a local smith (if you're in the bay area or sacramento I might be able to suggest a couple) who can check headspace or gas port size or send it back.

 

[Ballista One]

Re: AR10 Pressure Issues w/ Factory Ammo

EVERYTHING points to a bad barrel. NOTHING outside of the parts that form the chamber (bolt, barrel extension, barrel) can contribute to, or solve, the problems (extraction, blown primers) you are describing.

 

[Grump]

Re: AR10 Pressure Issues w/ Factory Ammo

The only thing headspace can do to chamber pressure is some small change in pressure.

Cases getting rims ripped off in a semi is a dwell time/timing/operating cycle thing which is affected greatly by gas port diameter, gas port location down the barrel, and the weight of the reciprocating parts.

All an off-center or off-axis chamber will *reliably* do is work the brass and make accuracy spotty if cases are not re-sized back to square head status.

IME and according to all the gas physics and metallurgy involved, softer brass can imitate high chamber pressure by (1) getting more "stuck" to the chamber (and not "springing" back), (2) giving bolt face markings and/or loose primer pockets at normal pressures instead of the usual 70,000 Piezo one source says it usually takes, and (3) not being strong enough at the rim to pull the sticky case out even if the operating cycle is NOT starting too soon.

Yes, with factory loads that are at safe operating pressures. Hornady has been one of them, Federal is the other, though that may be less true for more recent production.

The old rough chamber suggestion is one I've seen only once (ChiCom SKS), but the cases involved got marked and never failed as you describe. Remington brass, with a thicker extractor rim anyway.

I would be careful about concluding the chamber is tilted or off-center with anything that uses a spring-loaded ejector. The entire round is always pushed to the side of the chamber opposite the ejector, resulting in drag marks on the bullet after extraction. There's an educated/informed level of scuffing to recognize, but if the marks are way beyond that, then there may be a problem. But a short throat will affect pressure and extraction far, far more than mis-alignment will.

Keeping a bit mum until your results of working with the maker is proper. But more details about gas system and carrier components would help our collective guesswork...

Good luck on this. It can be far beyond annoying.

 

[squirrelsnpr]

Re: AR10 Pressure Issues w/ Factory Ammo

Grump-
Thanks for the information. I should have indicated I am getting the scuff marks on the bullet even when I drop them into the chamber and allow the slide to shut on the the round already in the chamber. I then carefully pull the round out, and apply pressure to counter act the force wanting to exject the round. Again, I am going to put anyone on blast here, b/c customer service has been the best I have ever expierenced! I am solely trying to figure out what may be causing this issue.

 

[pupdawg]

Re: AR10 Pressure Issues w/ Factory Ammo

I will say that it's a barrel problem.

Might be a short chamber or wrong gas hole size. I'd bet if you replace the barrel your problem goes away.

Since you don't want to "put anyone on blast" why don't you just ask the company with the best customer service if you can send your upper to them for a new barrel?

 

[zx1963]

Re: AR10 Pressure Issues w/ Factory Ammo

As someone said could be overgassed, also could be a short throat which can cause pressures to go up....