AR10 - Jammed while in battery

After running 15-20 rounds with no issues, the rifle jammed while in battery. I couldn't pull the charging handle back to save my life.

Dropped the lower receiver and could see that the bolt's locking lugs were partially disengaged. I made a half hearted attempt to pry the carrier back but thought better of it. Finally, after a little web searching, I settled on hammering the carrier back by striking it on the ring at the rear with a cold chisel. I had to hit it a number of times, not Arnold Schwarzenegger hammering, but with a fair amount of authority.

Eventually the bolt disengaged and the fired brass fell out, along with the loose primer.

Pictured is the mangled brass and an undamaged piece that was fired in the same round. A cursory visual inspection of the rifle shows no damage, but I haven't disassembled the BCG yet.

What could have caused this?

My first thought is did the bullet leave the barrel?

Looks like extreme overpressure, IMO.

Either a badly overcharged round, wrong powder, or the bullet was driven back into the case during feeding.

Excessive headspace, not fully in battery maybe.

That looks like extreme over pressure to me as well, looks like the primer on the round to the left is cratering as well doesent it? Are these factory or reloads?

If it's reloads, that's my first guess.

You check the headspace on that gun? Looks like some gasses flowed past the front if the shoulder as if it wasn't 100% in battery.

How does the bolt face and firing pin support look? Is there an air gap around the firing pin?

What's the protrusion reading?


As Ed mentioned bullet setback ban cause this. The feed ramps and barrel mouth are usually the culprits. Shortsapace can allow this to happen as can issues with firing pin support/protrusion.

Bullet left the barrel.

Handloaded, 43.6g of Varget, well within the safe range.

I've run maybe 500 rounds through the rifle before this so I suspect headspace should be OK. I do have the gauges, so I will check that, just the be sure. Bolt face and firing pin "look" OK, but I'll get a pic to show for your opinion.

Bullet driven back into the case during feeding - That has me thinking as I did get a double feed and this may have been the round that got jammed up into the round in the chamber.

I'll be back!

Thanks

DarrellA said:

Bullet left the barrel.

Handloaded, 43.6g of Varget, well within the safe range.

I've run maybe 500 rounds through the rifle before this so I suspect headspace should be OK. I do have the gauges, so I will check that, just the be sure. Bolt face and firing pin "look" OK, but I'll get a pic to show for your opinion.

Bullet driven back into the case during feeding - That has me thinking as I did get a double feed and this may have been the round that got jammed up into the round in the chamber.

I'll be back!

Thanks

Click to expand...

Double feeds are usually a magazine issue, but it can be a symptom of overgassing/bolt velocity issues.

Here's a pic of the bolt face. Yikes! Is that an imprint of the case head around the firing pin? I didn't notice that at first.

I am running Heavy Buffers AR10R-XH Buffer and Spring, maybe the bolt is still cycling too fast giving me the double feed?

Firing pin and bolt face support looks good. There is no void between the firing pin and bore in the bolt face.

If there is a void certain primers and loads will flow into the void and rip primers out or allow the primer to rupture. Not good.

The primer impressions are pretty bad for 500 rounds. Something is wrong with the ammo in that gun, or something else is going on.

There may not be enough preload on the buffer spring or headspace is short. A Tubbs .308 will help with the preload. Overgassing and short throats will cause the primers to try and pop causing that bolt face damage too. Adjustable Gas or lengthening of the throat would be things to look at.

That flattened and shiny area next to the extractor cutout tells me that bolt may be done. That is usually what I see when there are micro cracks in the corners of the bolt face.

GUNSnROTORS said:

Excessive headspace..

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After fireforming hundreds of rounds in rifles in which headspace is not standard, I'd have to rule that out.

Firing a cartridge in a chamber with excessive headspace (but not so excessive it won't fire at all) will typically cause a case head separation or partial separation and primer flattening without showing other pressure signs.

...not fully in battery maybe.

Click to expand...

Possibly, but the OP states the bolt lugs were at least partly engaged when he looked.

This partial engagement condition could be explained by the rifle beginning to unlock, but then excessive pressures almost immediately seizing the brass in place and sticking it tight in the chamber before it could fully unlock. Also, the very small unsupported area that expanded seems in keeping with normal operation - there is a narrow gap between the end of the barrel and the bolt head that would provide brass under pressure a place to go. There is also the location of the 'belt '(narrow unsupported area) being well into the solid portion of the case head.

If the bolt lugs were engaged at all and the bolt stayed closed, any out-of-battery condition would have had to have been extremely small. Equal to or less than the lead-in slope on the lug faces. If the lugs were only partly engaged prior to/upon ignition, I would expect to see some compression and/or chipping of the back corners of the bolt's locking lugs.

OP - how do the back of the bolt lugs look?

We had a AR-10 in .260 that did fire out of battery and it was pretty obvious. Split the upper, damaged the bolt, ejected the mag guts, blew the side of the case head completely out. Caused by a loose primer that came out of the pocket while feeding and got pinched sideways between the case head and bolt, and somehow ignited the powder charge when the bolt slammed into it.

NEVER use cases with loose primer pockets, especially in a self-loader.

TrappedinMD said:

That looks like extreme over pressure to me as well, looks like the primer on the round to the left is cratering as well doesent it?...

Click to expand...

A cratered primer without losing the edge radius on the primer does NOT equal high pressure. Cratering in the absence of the primer edge starting to flatten is typically a slightly oversize firing pin bore, a slightly undersize firing pin or a weak firing pin spring.

clandestine said:

If it's reloads, that's my first guess.

Click to expand...

Aye, but I don't think you can get enough Varget into a .308 case to do that. Even 45 grains is up into the neck, and that's just barely getting into high pressures with most rifles.

You check the headspace on that gun? Looks like some gasses flowed past the front if the shoulder as if it wasn't 100% in battery.

Click to expand...

Varget does leave cases dirty. Starts off slow and often smokes the case necks and shoulders at least a little, but that IS an odd pattern on the shoulder. I wonder when in the pressure cycle that occurred...

If the primers were not correctly seated (0.003"+/- below the case head surface), a slam fire could have occurred, but then we're back to comparing "out of battery" with
"partly engaged lugs".

I'm back to "overpressure".

E.Shell said:

After fireforming hundreds of rounds in rifles in which headspace is not standard, I'd have to rule that out.

Firing a cartridge in a chamber with excessive headspace (but not so excessive it won't fire at all) will typically cause a case head separation or partial separation without showing other pressure signs.Possibly, but the OP states the bolt lugs were at least partly engaged when he looked.

This partial engagement condition could be explained by the rifle beginning to unlock, but then excessive pressures almost immediately seizing the brass in place and sticking it tight in the chamber before it could fully unlock. Also, the very small unsupported area that expanded seems in keeping with normal operation - there is a narrow gap between the end of the barrel and the bolt head that would provide brass under pressure a place to go. There is also the location of the 'belt '(narrow unsupported area) being well into the solid portion of the case head.

If the bolt lugs were engaged at all and the bolt stayed closed, any out-of-battery condition would have had to have been extremely small. Equal to or less than the lead-in slope on the lug faces. If the lugs were only partly engaged prior to/upon ignition, I would expect to see some compression and/or chipping of the back corners of the bolt's locking lugs.

OP - how do the back of the bolt lugs look?

We had a AR-10 in .260 that did fire out of battery and it was pretty obvious. Split the upper, damaged the bolt, ejected the mag guts, blew the side of the case head completely out. Caused by a loose primer that came out of the pocket while feeding and got pinched sideways between the case head and bolt, and somehow ignited the powder charge when the bolt slammed into it.

NEVER use cases with loose primer pockets, especially in a self-loader.
A cratered primer without losing the edge radius on the primer does NOT equal high pressure. Cratering in the absence of the primer edge starting to flatten is typically a slightly oversize firing pin bore, a slightly undersize firing pin or a weak firing pin spring.

Aye, but I don't think you can get enough Varget into a .308 case to do that. Even 45 grains is up into the neck, and that's just barely getting into high pressures with most rifles. Varget does leave cases dirty. Starts off slow and often smokes the case necks and shoulders at least a little, but that IS an odd pattern on the shoulder. I wonder when in the pressure cycle that occurred...

If the primers were not correctly seated (0.003"+/- below the case head surface), a slam fire could have occurred, but then we're back to comparing "out of battery" with
"partly engaged lugs".

I'm back to "overpressure".

Click to expand...

The reloading stuff, I'll take your word as gospel.

My advice is to tell people to troubleshoot with factory ammo to try and find the real problem. Even factory ammo can be problematic though. For instance, I have observed FGMM have trouble in gas guns.

I don't mess with reloading, but what I know for a fact, is that trying to help 99/100 reloaders troubleshoot their gun is often a losing battle.

I recall seeing a thread a while ago by the O.P. that had some pretty substantial ejector smears on the brass from reloads. Maybe it wasn't this rifle in question?

I like to troubleshoot gas guns with good Nato spec ammo, then go from there. The brass is often strong and crimps are often good.

clandestine said:

The reloading stuff, I'll take your word as gospel.

My advice is to tell people to troubleshoot with factory ammo to try and find the real problem. Even factory ammo can be problematic though. For instance, I have observed FGMM have trouble in gas guns.

Click to expand...

FGMM eventually had to develop a "7.62" branded load with an altered pressure curve to run in gas guns and the brass is still too soft...

I don't mess with reloading, but what I know for a fact, is that trying to help 99/100 reloaders troubleshoot their gun is often a losing battle.

Click to expand...

Properly handloaded ammunition is equal to or better than factory ammo in several potentially beneficial ways. Greater velocity and/or muzzle energy are not one of those ways. Savvy handloaders don't try to push past factory loads because factory ammunition is routinely loaded to SAAMI maximum.

What I suspect you often run into are handloads that loaded with the idea that 'more is better' and the handloader seeks to increase velocities over factory ammo. Other issues may be due to those who say that this is just blasting ammo and not worthy of the care it actually warrants. With handloading, due caution and attention to detail is all that keeps us out of trouble.

I recall seeing a thread a while ago by the O.P. that had some pretty substantial ejector smears on the brass from reloads. Maybe it wasn't this rifle in question?

Click to expand...

I missed those posts, but ejector marks are always bad news and a clear sign that the brass has been subjected to enough excess pressure to begin to shear material into the ejector recess.

I like to troubleshoot gas guns with good Nato spec ammo, then go from there. The brass is often strong and crimps are often good.

Click to expand...

Agreed, the NATO ammo is loaded to 7.62x51 pressures and is what most military style rifles are designed for. The .308's, as you know, run higher pressures on a slightly different curve and are not always compatible with 7.62 rifles.

I love reading threads like this guys, your expertise is to learn from.

Pics of the back of the lugs.

I did have a problem with ejector smears on another AR10 (Good recall!). On the suggestion that the gun was over gassed, I installed an adjustable gas block and that problem went away immediately.

I'm pretty cautious with reloading; I check the scale calibration and CBTO every 5-10 rounds. That's not to say I didn't F one up, but, that wouldn't be my first guess.

Occam's Razor - I'm fixating on that double feed jamming the bullet back into the case.

I have a few boxes of FGMM, I'll run them through and see what happens.

I haven't been overly pleased with my primers and/or primer seating lately, I don't know if it's a bad batch of primers or my skill level. And, I do get an occasional FTF that has me shaking my head. But of the maybe 5,000 rounds I've reloaded it's a very recent occurrence and only with this batch of primers. Only on my reloads, not with the FGMM.

Any recommendations for a replacement bolt, just in case this one does have some cracks? I'll get a replacement rather than risk using this one again. May even order up the Tubbs spring, that's a super easy swap out.

Thanks for all the input, you guys are the best!

OP. One question. Do you case gauge every round you've reloaded? I'm not sure if this can cause such a problem, but out of every 1K rounds I reload, I get 4 or 5 rounds that refuse to case gauge. I've gone through the trouble of pulling the bullet and primer, resizing the brass, and reloading. Success rate for that runs about 50%.

I second the pushed back bullet from the double feed. That would have increased the pressure to where the brass flowed into the ejector and extractor recesses which locked up the bolt so it couldn't unlock completely. An OOB detonation would have blown through the unsupported part of the case and I see not evidence of that.

The brass stretching started on ignition but the pressure dropped after the bullet passed the muzzle. Because the brass had already flowed prior to the bullet passing the gas port, the normal gas pressure, or even the momentary additional pressure was still not enough to operate the bolt fully.

Just a SWAG.

outrider58 said:

OP. One question. Do you case gauge every round you've reloaded? I'm not sure if this can cause such a problem, but out of every 1K rounds I reload, I get 4 or 5 rounds that refuse to case gauge. I've gone through the trouble of pulling the bullet and primer, resizing the brass, and reloading. Success rate for that runs about 50%.

Click to expand...

No, I don't. Actually, never have! Except for the earlier AR10 that had the ejector swipe problem, I was checking rounds when trying to track down that problem, which was solved with an adjustable gas block.

Is that a standard practice?

DarrellA said:

No, I don't. Actually, never have! Except for the earlier AR10 that had the ejector swipe problem, I was checking rounds when trying to track down that problem, which was solved with an adjustable gas block.

Is that a standard practice?

Click to expand...

It is with me. I don't think everyone does. I load so many, I figure chances are, I'll have a problem if I don't.

YMMV

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