AR10: Occasional Jam

[rg1911]

I'm getting an occasional jam in my Armalite AR10 in .260Rem. This is particularly annoying when it happens during a match.

The bolt goes back far enough to catch and chamber a round, but the fired case is not ejected. (images are a recreation of what happens)

The equipment is:

- Armalite AR10
- Armalite 10-round magazines
- Krieger barrel, .260 Rem
- Cases are surplus RA64 cases that are resized and neck-turned. (I got tired of losing Lapua brass. Problem also happens with Lapua brass.)
- Buffer tube with 3 tungsten weights
- SLR adjustable gas block, open almost all the way
- Thunderbeast silencer, direct-thread
- Load: Berger 140 VLD, 41.6 gr H4350, Federal GM Match LRP, 2720fps (Oehler 35P chrono)

Cases land anywhere from 20 feet away to on the bench.

The buffer tube weights are necessary to slow the bolt sufficiently that the case has time to shrink back down enough that the rim is not torn off. (This is caused by the requirements I gave to to my 'smith. He recommended that I really didn't want what I was asking. Now I know why.)

About 75 percent of ejected cases are dented at the mouth:

Primers show no indication of an excessive load (I know primers are not the best way to tell, but they do give some idea):

Because the ejection/feed problem is intermittent, I have not opened up the gas block any more, since it already is almost all the way open.

Thank you for any suggestions I should try.

Richard

 

[Mattrmvpd]

Check buffer and spring.
check gas rings on bolt

had a similar issue and turned out to be the buffer spring needing to be tuned to run perfect.

 

[BCP]

You needed to open the gas block all the way to get it running? 20ft is a loooooong ways.

 

[TonyTheTiger]

You needed to open the gas block all the way to get it running? 20ft is a loooooong ways.

My first thought as well.
Also curious what requirements were given that the smith wasn't keen on.

 

[newguy2k3v2]

What length barrel and gas system?

I’ve had some troublesome large frame guns. Make sure the chamber is clean. I’d try an extractor and/or spring/o-ring. Possibly a slash xh buffer to help slow the bcg down more and delay unlocking a little.

 

[Snuby642]

You are running your gas block ass backwards an amongst other things beating the shit out of brass and the gun.

Close the gas block all the way and back it out 1 turn.

Adjust like the manufacturers tell you. Full open with a can is not going to work.

 

[bfoosh006]

I will second the turn the gas block closed more.

Your AR10 could be cycling to fast, and not allowing the fired brass enough "time" to get out of the action.

Try the "single , mag fed round test" at the lowest setting it will function 100% of the time.

Single Mag fed round, adj. the the GB until the BCG locks back 100% reliable, then adj. slightly more open.

 

[Snuby642]

I would like to add, do the test and adjustments with the can on if you are going to run it.

 

[powdahound76]

In guessing a tight match chamber is what the smith tried to convince him to do another way?

Too snug on a gasser with a can.

Should not need much open on GB.
did you go +2 on gas or what length?

Pull barrel. Sheepishly ask smith to open neck.

then do gas block adjustment as noted.

 

[Shoots 700's]

Im thinking with the can its running too fast. Close the gas port and adjust until it runs correctly

 

[CCFRN]

I had a problem with extraction/ejection in my AR-10 pattern rifle.

It turned out to be too much extractor tension. My bolt had Two O-Rings and Two extractor springs. I removed the O rings and the small inner extractor spring. It completely fixed my erratic extraction/ejection problem. Hope this helps you.

 

[Greg Langelius *]

Couple of ideas to ponder.

I have repeatedly suggested to this forum that the best way to utilize an experienced Gunsmith is to set a goal/task to accomplish, set a budget, and talk it all out before the tools start running. The Smith has probably done what you need done many times, and has some good ideas about what does and doesn't work so well. That's even more true with AR's than with Bolt guns.

This topic is a golden example of that concept.

Putting it plainly, when the two of you sit down to discuss this, there's usually only one professional in the room, and it's usually not the customer. In matters where opinions differ, the experienced Smith's opinion is usually the superior one. Your opinion obviously was not. Any time you're departing from customary design practice, you need a really good reason for doing so. These aren't Legos we're building here.

I don't have a .260 chambered AR, but I have several AR's chambered in 5.56, a pair in 6.5G. and one in .308. I've been shooting the 260 in a bolt gun for just about two decades.

Looking at that fired case base and primer; that cartridge is loafing, just barely making working pressure. I'd try upping the load in .2gr-.3gr increments, and doing nothing else. I expect the cycling will improve. My 140gr 260 load is just under 44gr of H-4350. It's hot,; so don't go there without serious testing first. Be aware that some AR's don't like such pressures, which is why you should work up carefully. Consult a good handloading guide about how to read pressure signs on fired cases. My load could be too much for your gun, but that fired case says you can go higher than what you're using currently. I'm currently changing my load to include 140gr Speer Gold Dots and H-4831SC. It's lower in pressure and safer for my rifle, but I'm also using a 28" bolt gun and my AR's tend to prefer the faster powders.

I hope you get your rifle sorted out and really would like to find out how this all turns out; so please post your outcomes.

Greg

 

[rg1911]

powdahound76 is correct; I specified a match chamber that would have been fine in a bolt rifle. I knew I would have to neck turn almost all case brands. Since I had had no problems with my AR-15s, I figured (incorrectly, as it turned out) that an AR10 would be the same. Live and learn.

The bolt speed has been adjusted by substituting tungsten weights in the buffer. I may swap out one more weight.

I also bought and installed a JP spring. At the range, the first round jammed, but the next four magazines' worth functioned without a single hiccup. I do not view 25 rounds as proof that the problem is fixed, so will see how the rifle continues to function.

I would like to thank everyone for your thoughtful advice. Should the rifle malfunction again, I will refer back to this thread and delve into the not-so-easy fixes.

Cheers,
Richard

 

[Jigstick]

I can’t even fucking imagine neck turning for a gas gun. With the volume of rounds being put through mine. Zero time to fuck with that. That tight neck is increasing pressure. Compound that with the suppressor. You’re BCG has to be cranking. Set your iPhone on slow motion and record a couple shots at the ejection port. You’ll get a lot of answers quickly

 

[Jsp556]

@sinister

 

[sinister]

I'm getting an occasional jam in my Armalite AR10 in .260Rem. This is particularly annoying when it happens during a match. [You wouldn't want this during a hunt or combat]

The bolt goes back far enough to catch and chamber a round, but the fired case is not ejected. (images are a recreation of what happens)

- Armalite AR10
- Armalite 10-round magazines
- Krieger barrel, .260 Rem
- Cases are surplus RA64 cases that are resized and neck-turned. (I got tired of losing Lapua brass. Problem also happens with Lapua brass.)
- Buffer tube with 3 tungsten weights
- SLR adjustable gas block, open almost all the way
- Thunderbeast silencer, direct-thread
- Load: Berger 140 VLD, 41.6 gr H4350, Federal GM Match LRP, 2720fps (Oehler 35P chrono)

Cases land anywhere from 20 feet away to on the bench.

The buffer tube weights are necessary to slow the bolt sufficiently that the case has time to shrink back down enough that the rim is not torn off. (This is caused by the requirements I gave to to my 'smith. He recommended that I really didn't want what I was asking. Now I know why.)

About 75 percent of ejected cases are dented at the mouth:

Primers show no indication of an excessive load (I know primers are not the best way to tell, but they do give some idea):

Because the ejection/feed problem is intermittent, I have not opened up the gas block any more, since it already is almost all the way open.

Thank you for any suggestions I should try.

How long is your barrel? Standard 20-inches? Is this a rifle or carbine gas system?

Dave Tubb did tons of SR-25 research in his march towards 12 NRA Highpower Rifle Champion titles. One of the things he had to trouble-shoot is the problem you've encountered with a .260 / 6.5-08 -- your cases are still obturated against your chamber walls while the carrier is in unlock and primary extraction. He fixed this by going to a gas tube two inches longer than standard rifle ( a "+2" gas tube). A chamber requiring neck-turning gives you no allowance for fouling or expansion as it heats. Your system is already over-gassed.

You're further complicating your pressure-time curve by adding a suppressor -- your port pressures are going to still be high while the suppressor holds gas, acting like a longer barrel, until the projectile leaves the tube -- "Uncorking").

Close your gas block setting and try an "XH" stainless steel buffer body as well as the tungsten weights -- give the gas and the longer pressure curve work to do -- pushing a total heavier buffer and carrier weight -- it'll slow down your cycling to give the ejector(s) time to ditch the empty overboard. You can see the majority of your cases are hitting the left-hand shooter "Ejector bump" rather than ejecting before slamming into the hump or the aft end of the ejection port. They should land about three feet away (instead of casting them into the next county like a G3 or AK).

Gas (and the fouling left behind as they cool) only has two places it can go -- out the muzzle or end of your suppressor, and into your bolt carrier and your upper. Any that gets on to the top round is going to go back into your chamber, dropping the number of rounds you'll be able to get out before requiring the gratuitous forward assist tap because of the tight neck.

 

[Earnhardt]

How long is your barrel? Standard 20-inches? Is this a rifle or carbine gas system?

Dave Tubb did tons of SR-25 research in his march towards 12 NRA Highpower Rifle Champion titles. One of the things he had to trouble-shoot is the problem you've encountered with a .260 / 6.5-08 -- your cases are still obturated against your chamber walls while the carrier is in unlock and primary extraction. He fixed this by going to a gas tube two inches longer than standard rifle ( a "+2" gas tube). A chamber requiring neck-turning gives you no allowance for fouling or expansion as it heats. Your system is already over-gassed.

You're further complicating your pressure-time curve by adding a suppressor -- your port pressures are going to still be high while the suppressor holds gas, acting like a longer barrel, until the projectile leaves the tube -- "Uncorking").

Close your gas block setting and try an "XH" stainless steel buffer body as well as the tungsten weights -- give the gas and the longer pressure curve work to do -- pushing a total heavier buffer and carrier weight -- it'll slow down your cycling to give the ejector(s) time to ditch the empty overboard. You can see the majority of your cases are hitting the left-hand shooter "Ejector bump" rather than ejecting before slamming into the hump or the aft end of the ejection port. They should land about three feet away (instead of casting them into the next county like a G3 or AK).

Gas (and the fouling left behind as they cool) only has two places it can go -- out the muzzle or end of your suppressor, and into your bolt carrier and your upper. Any that gets on to the top round is going to go back into your chamber, dropping the number of rounds you'll be able to get out before requiring the gratuitous forward assist tap because of the tight neck.

Now this is a great post!
Thanks!