AR-10 Malfunction Advice - got some?

[RainDodger]

Second update - Installed the Odin adjustable gas block AND the heavy buffer. I initially set the gas block closed, which of course means that the BCG did not cycle at all. I opened it in stages until I had the bolt reliably locking back on an empty magazine. (The Odin gas block has 26 detents available for adjustment, which makes it easy to tell how far out you've backed off the screw.) I was getting a reliable BCG lock-back at 7 clicks open. I fired 8 rounds with no problems encountered.

We adjusted my buddy's identical rifle and it performed the same. He fired 10 rounds of factory .308 rounds with no malfunction. Considering neither of us could get more than 2 rounds fired without a problem, things are looking up. We will take both rifles to the range on Monday for a thorough test. Although I'm not yet confident, I think we're on the road to solving the problem.

(I would have done some more shooting here at home, but some distant neighbor came over to my driveway and got kind of confrontational, despite the fact that it is totally legal to shoot where I live in the unincorporated county. The guy wouldn't tell me where he lives, but complained that he's had "stray shots" around his property in the past. (definitely not from my shooting!) He said that one of his neighbors called and said that he was hearing "explosions"... I told him nope, it was just a .308. I keep my close neighbors aware of what I'm doing and when, and this guy wouldn't tell me where he lives or give me a number so I could tell him if I was shooting. As you can tell, he pissed me off, but I try to keep neighbors happy, so I kept the shooting session short.)

On a different note... a couple of you said that you felt my hand loads were too hot. Well, I took another look at the books. According to Lee's hand loading manual, with a 155 grain jacketed bullet, the minimum load for IMR-4064 is 43 grains, and the max is 47. (I didn't include tenths, to keep it simple) My load is at 45.0 grains, so I don't view it as too hot. There are no signs of any over-pressure or problem with them. In deference to the opinions above though, I DID load some starting loads that are at the very low end, and I'll shoot them on Monday for grins. (I loaded some at 42.0 grains, which is a starting load in a different manual, but below a starting load in Lee's book.)

If anyone's interested, I'll let this thread know how the range session goes on Monday.

 

[Gordon]

why I ordered an adjustable gas block at the outset of my new AR-10 build. The AR10 is not the little .223 case and uses twice as much propellant . I could see why it needs to be throttled back with direct impingement

 

[MistWolf]

The 308 also has a larger bore. The gasses have greater volume to fill.

 

[Gordon]

The 308 also has a larger bore. The gasses have greater volume to fill.

Still even volume has to be managed. I know all too well with FAL and others , anyway just my feelings and seeing other's AR10 issues. FWIW the DPMS factory rifles usually run very well with factory ball ammo as do high end AR-10s with no gas block corrections. Maybe like M-14s they have problems with slower powders and different bullets. I know I have to fiddle with FAL gas adjustments when I use such loads.

 

[MistWolf]

I agree that no matter what caliber, gas has to be managed, or as I've been saying, the gas drive has to be right.

My point is that while the 308 generates greater mass of gas, because the 308 bore is a larger diameter than the 5.56, it has more volume to fill which affects the pressure curve.

My experience with the MP-10 is that it's ported for commercial pressure ammo. Use of NATO pressure 7.62 all too often results in short stroking, especially when it's cold. If my plans to suppress the MP-10 come to fruition, I'll probably be looking into an adjustable gas block for it.

The problem with M14s (and Garands) is that using the wrong bullet weight & powder results in bent op-rods. The AR, having no op-rod with its piston, is a more versatile system. The FAL doesn't seem to have problems with different bullet weight s & powder, but I think that's due to its op-rod being straight.

 

[Gordon]

FAL has an adjustable gas block

 

[Slamfire]

The problem with M14s (and Garands) is that using the wrong bullet weight & powder results in bent op-rods.

Agree. If you over accelerate the bolt in a M1 Garand you will bend the operating rod. It is very long and not very stiff. The M1a operating is stiffer, you will bend the saddle section under the barrel with repeated over pressure loads, if you don't bust the welds on the rod. You will also have malfunctions and bolt over rides as the bolt travels faster than the cartridge can rise from the magazine.

And this one. Never mentioned, but it is real. You will crack the heel of the receiver. The rear of the receiver is where the bolt hits and rebounds. Hit it hard enough, long enough, and it will crack:















A poster found this in the snow. No idea how it got there!



When you are having function issues with a gas gun, the absolute first thing to do is cut your loads.

 

[MistWolf]

FAL has an adjustable gas block

Yes. Yes it does. Still, part of the reason the FAL is more forgiving of ammo than the M14 is that the op-rod is straight and doesn't get bent. The AGB controls gas flow, but not the pressure curve. It's the pressure curve that damages M14 and Garand op-rods.

 

[MistWolf]

When you are having function issues with a gas gun, the absolute first thing to do is cut your loads.

That depends on the design. The most common AR malfunctions can be traced to a weak extractor spring.

 

[RainDodger]

Range trip today and the results are a bit weirder than I thought they'd be! Remember, we have 2 identically-built rifles. Prior to the range trip I adjusted the gas blocks so we would have a locked back bolt on the last round.

Both rifles malfunctioned identically. We would have between one and four malfunctions in 20 rounds. Just as first described in post #1. As described, this was happening with all the magazines. All newly-bought Magpul 20 round magazines, and in both rifles.

I was at a gun show over the weekend and there was a dealer there selling the new Hexmag magazines for a good price. I bought one 20 rounder to try. We were getting ready to leave and I thought to try the Hexmag just for grins. I went through 20 rounds with zero malfunctions. I handed it to my buddy and he proceeded to put 40 rounds through his rifle with no malfunctions.

I don't think I've heard of something like this.... 2 rifles, neither one liking multiple Magpul 20 round magazines. Now we're going to pick up a few more Hexmags and try them.