AR-10 Malfunction Advice - got some?

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RainDodger

RainDodger

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Hi, guys -

I've built many AR-15s with no functioning issues and am quite familiar with that platform. Just finished my first AR-10 (DPMS pattern) build. Mid-length gas system, carbine stock with a standard 7" buffer tube with standard AR-10 carbine spring and buffer. Gas block is a standard non-adjustable type.

I'm getting malfunctions where a round is half-way to the chamber (nose up) and the bolt has ridden over the case and is jammed against the side of the case. Bolt is locking back on last round and it's cocking the trigger adequately - I would guess there's definitely enough gas.

My guess is, I should try a heavier buffer to slow things down, but I know there's guys out there who have more knowledge than I do.

I'm not seeing a large array of buffers out there though... what's the recommendation from those who know? :)
 
Sounds like a possible magazine problem as it had started to strip the next round but failed to come completely out of the magazine and the bolt is hitting halfway up the cartridge if I'm understanding correctly. I would try a different magazine and ensure the magazines are completely seated on an open bolt and then let the BCG go without holding it back and see if you have any issues.
 
Forgot to say!.... new Magpul mags and it's happening on all (3 different) mags. I thought right away it was a magazine problem but I kind of eliminated that one. Thanks for the thought.

(Mags are seated completely and from the locked back position, the bolt will chamber a round perfectly. The malfunction happens on subsequent shots.)
 
RainDodger said:
Forgot to say!.... new Magpul mags and it's happening on all mags. I thought right away it was a magazine problem but I kind of eliminated that one. Thanks for the thought.
Click to expand...

Where does the round seem to hang up? On the feed lips of the upper/chamber, or does it not even get that far?

Does it happen only on a full mag, half a mag, one round in the mag?

Does the upper have m4 feed ramps?
 
If you can rule out magazine issues, and feed ramp issues, then likely your faced with what your initial hypothesis is and start working on the buffer, buffer spring and gas port. Also, I would make sure the BCG is lubed well, but I'm sure you've done that as you've assembled quite a few AR15's.
 
Yeah, that's what I figured, thanks. In looking around, I'm not seeing any great array of different weights for .308 buffers though. I really don't want to spend a ton of $ on some aftermarket setup like J&P sells through Brownell's. I suppose I could put on an adjustable gas block also. A buffer is just easier though, if I can find one to try!
 
I just ordered a 5.5 oz. (stock carbine buffer for .308 carbine is 3.8 oz.) buffer to try, from Primary Arms. I talked to a buddy that had a similar problem and this solved it. I will let this thread know of the results after testing, so anyone else suffering the same issue might benefit.
 
Bolt-over-base is usually caused by the round stack not being presented to the bolt fast enough. That can be either because the mag springs are weak or the bolt is not cycling fully enough to let the magazine push the round stack up in time for the feed action. I dunno how a heavier buffer would help, unless it somehow slows the timing of the bolt's return to battery.
 
Does it lock back on an empty mag? My AR15 was doing this frequently and also failing to lock back due to short cycling. A lighter buffer solved it for me. Actually I just took the buffer apart and took the weights out.
 
OP; I had the exact same problem with a Stag AR-10 build. Tried an adjustable gas block (didn't work, just lost bolt lock-back) and different mags and different ammo. I originally was using the PMag 10's, and had the same problem with steel 10's, but a PMag 20 worked fine....or by that time I had just run enough rounds through it to loosen things up. So I wouldn't give up until you get at least 30-50 rounds through, and make sure to try different mags. There are, as some pointed out, 5.7oz buffers available and those might help but I didn't have to go that route. Someone else suggested a Tubb's Speedlock Flat Buffer Spring and I have had one on backorder for months, but that's another thought.
 
RainDodger said:
I'm getting malfunctions where a round is half-way to the chamber (nose up) and the bolt has ridden over the case and is jammed against the side of the case. Bolt is locking back on last round and it's cocking the trigger adequately - I would guess there's definitely enough gas.
Click to expand...


You did not state the cartridge nor the load. But, based on similar experiences with AR15's, cut your load.
 
Responses to above. Yes, the bolt always locks back fine, as I said. I can't imagine the mags have weak springs, as they're all brand new/never used 20 round Magpulls, but I've seen weird stuff before and will look around for a Lancer mag or some other brand to throw in the bag. My first thoughts are that it's getting a lot of gas and a heavy buffer will slow things down a bit. If that doesn't change things, I'll go the other way! All thoughts are welcome! It will probably take a week to get the new heavier buffer and then a week until I can get to the range, so my results will be a while.

On the good side, the 16" Ballistic Advantage barrel seems absolutely stupendous. Even with a tactical scope on there with a fairly thick reticle that blots out a bunch of the target, the rounds going downrange successfully were giving me about 1" groups, which I have a hard time doing, most of the time. So... once I get the cycling under control, this thing will be a great shooter.
 
Ah, Slamfire... the loads are 155 grain Nosler Custom Competition bullets over 45.0 grains of IMR-4064, COL is 2.800" and by my chronograph, doing 2,846 FPS. Not an overly hot load, and it functions perfectly in a Rock River LAR-8. I can certainly try something a little lighter though. Thanks. (oh, that was 2,846 out of a 26" barrel, not this 16")
 
If you chamber / extract / eject the first round without issue but the follow-up round acts as described, I'd first try a heavier buffer. Sounds like the bolt is cycling excessively fast relative to the speed of the magazine spring. Additionally, very carefully check the inside of your upper for any burrs, machine marks, etc which could cause this. Give it a very thorough cleaning and run it wet. Not stupid wet but perhaps wetter than is your norm. Glistening but not running.
 
Try a couple of different rounds, just to make sure you aren't just dealing with something the gun doesn't like. As far as lube, AR-10's seem to be more sensitive (in most cases) to lack of lube than AR15's. I recommend lubing the BCG and upper receiver walls with white lithium grease from auto zone. It works well, it's cheap, and doesn't evaporate or fly out of the gun when you fire.
 
RainDodger said:
Forgot to say!.... new Magpul mags and it's happening on all (3 different) mags. I thought right away it was a magazine problem but I kind of eliminated that one. Thanks for the thought.

(Mags are seated completely and from the locked back position, the bolt will chamber a round perfectly. The malfunction happens on subsequent shots.)
Click to expand...

Back the truck up a second. WHICH mags, are they 30 rounds or more? The reason I ask is I had a similar issue with
some high capacity mags. Do you get the same FTF with 20 round mags?
 
These are 20 round mags. Thanks for the tips, guys. I'm going to go one step at a time so I know what the problem is, should I fix it. I'll let this thread know what I find! Onward, to further testing... :)

Roger
 
FL-NC said:
I recommend lubing the BCG and upper receiver walls with white lithium grease from auto zone. It works well, it's cheap, and doesn't evaporate or fly out of the gun when you fire
Click to expand...
It's also a dust magnet. CLP is the lube of choice because it has adequate viscosity and film strength to be useful as a lube while being runny enough (technical term, that) to migrate the spooge (grit + lube = spooge) out from in between close-tolerance moving bits.
 
rbernie said:
It's also a dust magnet. CLP is the lube of choice because it has adequate viscosity and film strength to be useful as a lube while being runny enough (technical term, that) to migrate the spooge (grit + lube = spooge) out from in between close-tolerance moving bits.
Click to expand...
That's not what we found in afg., which is pretty dusty. Yes, dust gets in the grease- also the nasty powder residue (when you actually fire the weapon). BUT- its GREASY dust and residue. The grease continues doing its job, because it remains greasy and it remains in the weapon (unlike CLP, which evaporates and flies off and ends up on the outside of the weapon, and on your equipment, uniform, glasses, etc. where it does no good)- even if it is a machine gun. The difference is it just changes color, and when you clean the weapon after the fight, the grese and nastiness comes out fairly easy. Use it and fire a few hundred rounds of wolf in an AR.
 
Last edited:
You're overgassed. The exact problem you describe happens often when .308 ARs are suppressed that run fine otherwise. The fix is an adjustable gas block, pretty cheap'n'easy.

FL-NC said:
That's not what we found in afg., which is pretty dusty. Yes, dust gets in the grease- also the nasty powder residue (when you actually fire the weapon). BUT- its GREASY dust and residue. The grease continues doing its job, because it remains greasy and it remains in the weapon (unlike CLP, which evaporates and flies off and ends up on the outside of the weapon, and on your equipment, uniform, glasses, etc. where it does no good)- even if it is a machine gun. The difference is it just changes color, and when you clean the weapon after the fight, the grese and nastiness comes out fairly easy. Use it and fire a few hundred rounds of wolf in an AR.
Click to expand...

Valve grinding compound is greasy, too....
 
MachIVshooter said:
You're overgassed. The exact problem you describe happens often when .308 ARs are suppressed that run fine otherwise. The fix is an adjustable gas block, pretty cheap'n'easy.
Click to expand...
What leads you to believe the rifle is over gassed?
 
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