New Armalite AR-10 Build: Ejection and Feeding Problem

[rg1911]

My 'smith finally got all the parts for my Armalite AR-10 in .260 Rem with a Krieger barrel. He ran a couple magazines of ammo through it for function testing (the Giessele trigger needed some tweaking to keep the rifle from doubling) and experienced no problems.

So I took it to the range Monday to work up some loads using new Lapua brass, H4350 and Berger 140-gr Hybrids. Of 80 rounds fired, at least 20 failed to eject. In most of those cases, the case had flipped so the base was pointing at the muzzle end. Of the cases that did eject, most dropped within 2 feet. Checking the cases, I noticed that each rim showed damage, as though the extractor was trying to take a chunk out of the rim.

I also wanted to single-feed the rounds, so I loaded one into my Armalite 10-round mag. When I dropped the bolt, the round jammed into the barrel below the chamber and shoved the bullet into the case.

Obviously, this rifle is not yet ready for our long-range matches.

I searched the forum and did not find another mention of this problem. One post did say that the poster had to full-length size new Lapua brass to get it to chamber and extract.

Does this sound like the cause of my ejection problem? (I had only run the cases over an expansion mandrel -- necks were really tight -- and then neck-sized them.)

Any ideas about the feed problem?

Thank you,
Richard

 

[rg1911]

Update:

Looks like the two problems have been found.

Feeding: The shape/tip of the Berger 140-gr Hybrid was ramming into and stopping on the bottom edge of the chamber, just at the spot where the chamfer starts. A different bullet shape feeds just fine. The solution is to very slightly increase the chamfer at the spot where the bullet is catching.

Ejection: The *probable* cause (still need to verify) is a too-tight neck. The chamber neck is .297. The neck of a loaded round using Lapua brass is .294. The neck of rounds loaded using Remington cases is .292, and these eject just fine. I'm deciding whether it's better to turn the necks of my Lapua cases or have the chamber neck opened up to .299.

Cheers,
Richard

 

[sakobob]

turning necks on a gas gun is a lot work for brass you may lose after one firing, it gets old looking for that 1 or 2 pieces
of brass hiding in the weeds. not to mention costly lapua.
and if your in a match where your not going to have time to police your brass. . . . well your S O L.
something to think about.

 

[Oryx]

Why would a tight neck cause the case to eject improperly after it was already extracted from the chamber? I would think you would see feeding issues prior to ejecting issues with a tight neck in a semi auto, no?
Also, are you positive you have the correct spring and buffer weights and gas system is not causing over/under pressure issues? Much easier to check those than monkey with turning necks and modifying chamber. Just a thought.
Does your gunsmith have a lot of experience with large frame AR's in different calibers? I'm not saying that he doesn't know what he's doing, but many just throw parts together like a standard AR thinking everything is built to a mil spec (not the case with the larger frame ARs). You and he may have already considered and discussed this - I'm just asking for clarification.
Good luck with getting it running right - it sounds like a great set up

 

[kd_nevada]

Re ejection, have you tried lower powder charge? If the case is sticking, and the extractor is doing a wonder to the case rim, I'd look for other pressure signs. How does it shoot so far?

 

[silver78]

(the Giessele trigger needed some tweaking to keep the rifle from doubling)

Which model trigger did you use? FWIW when I upgraded my AR10 I selected this trigger.

Hi-Speed National Match - Match Rifle Trigger

"The Geissele Hi-Speed Service Rifle Trigger is designed
for NRA High Power Service Rifle competition. It is an
approved trigger for use in CMP sanctioned competitions
such as: National Trophy, NTIT and all Excellence In
Competition matches. Service Rifle competitions require
military based weapons to meet certain criteria, one of
which is a minimum trigger pull weight of 4.5 lbs. "

With the heavier recoil of a 308, I was concerned that the lighter pull triggers like the DMR would double.

 

[rg1911]

Which model trigger did you use? FWIW when I upgraded my AR10 I selected this trigger.

Hi-Speed National Match - Match Rifle Trigger

With the heavier recoil of a 308, I was concerned that the lighter pull triggers like the DMR would double.

Mine is the same trigger. Got it from Midway:

Geissele Match Rifle Adjustable Trigger AR-15 LR-308 Small Pin .154

Tweaked it a very tiny bit and it hasn't doubled in another 100 rounds.

So far, I'm very pleased with the trigger.

Richard

 

[rg1911]

Re ejection, have you tried lower powder charge? If the case is sticking, and the extractor is doing a wonder to the case rim, I'd look for other pressure signs. How does it shoot so far?

There was no difference between the starting load (39.5 gr H4350) and the max load (41.5 gr H4350).

I'm still working on finding the optimum load with the Berger 140-gr Hybrid, but the rifle has shown that it can achieve .5 MOA at 200 yards.

I do intend to check with the Sierra 142-gr MK and the Lapua 139-gr Scenar bullets I have on hand.

Richard

 

[rg1911]

turning necks on a gas gun is a lot work for brass you may lose after one firing, it gets old looking for that 1 or 2 pieces
of brass hiding in the weeds. not to mention costly lapua.
and if your in a match where your not going to have time to police your brass. . . . well your S O L.
something to think about.

Understood. However, I'm old and have had major back surgery that forced me to quit NRA Highpower competition. I can't get into (or out of) position anymore. So I restrict myself to matches that let me shoot from the bench. A trifle depressing, but it beats not shooting at all.

Richard

 

[samnev]

I put a Geissele SSA-E trigger in my AR10A4 and not had a single problem. It greatly improved my groups compared to the very heavy trigger pull on the stock trigger.

 

[sinister]

The 260 Remington (6.5-08) in an Armalite AR-10 (or AR-based auto-loader) is a sensitive and problematic beast. It does NOT have the same pressure curves over time as 308/7.62mm.

Armalite's factory offerings appear to be 308/7.62 and 243.

Why is the gunsmith you mentioned as builder not helping you with this? What brand of ammo or load was he using since he didn't have problems or stoppages?

I believe Dave Tubb extended the length of the gas system on his rifle by two inches when he was shooting the SR-25 to address this issue

 

[rg1911]

Why would a tight neck cause the case to eject improperly after it was already extracted from the chamber? I would think you would see feeding issues prior to ejecting issues with a tight neck in a semi auto, no?
Also, are you positive you have the correct spring and buffer weights and gas system is not causing over/under pressure issues? Much easier to check those than monkey with turning necks and modifying chamber. Just a thought.
Does your gunsmith have a lot of experience with large frame AR's in different calibers? I'm not saying that he doesn't know what he's doing, but many just throw parts together like a standard AR thinking everything is built to a mil spec (not the case with the larger frame ARs). You and he may have already considered and discussed this - I'm just asking for clarification.
Good luck with getting it running right - it sounds like a great set up

Thank you for the extra things to check.

Fortunately, my 'smith is well-known in my area for his experience and expertise with semi-autos both large and small. He's successfully built several AR-10s in 260 Rem. I've been giving him my work for many years.

I just prepared 10 rounds with necks turned so the loaded neck measures .292 -- the diameter of the loaded Remington rounds the 'smith used for function testing. The theory is that the bolt has started to cycle before the case neck has completely loosened from the chamber. This would not be a problem in a bolt rifle because there would be plenty of time for the case to contract before the bolt was operated. For my match bolt rifles, I have the neck area only .0015 over the diameter of a loaded round and have to turn all my brass.

If the rifle functions properly with the test rounds, I'll decide whether to continue turning the necks on all my Lapua cases; switch to a brand that has thinner neck walls; or just ream out the neck another .002.

Richard

 

[rg1911]

The 260 Remington (6.5-08) in an Armalite AR-10 (or AR-based auto-loader) is a sensitive and problematic beast. It does NOT have the same pressure curves over time as 308/7.62mm.

Armalite's factory offerings appear to be 308/7.62 and 243.

Why is the gunsmith you mentioned as builder not helping you with this? What brand of ammo or load was he using since he didn't have problems or stoppages?

I believe Dave Tubb extended the length of the gas system on his rifle by two inches when he was shooting the SR-25 to address this issue

Armalite does offer an AR-10T upper marked 260 Remington, so I assume they know that a number of people build rifles in that chambering.

My 'smith is helping me. Turning the case necks was my idea. I'd rather start with something simple that does not permanently modify the rifle. If the problem is caused only by the thicker necks of the Lapua cases, I'd rather turn a few necks than recut the chamber neck so it works with unturned Lapua cases but becomes sloppy with Remington factory ammo.

Concerning adding 2 inches to the gas system, that probably isn't possible. Krieger drills the gas port before shipping the barrel.

Richard

 

[sinister]

You can try a Tubb Carrier Weight System or an XH buffer -- these individually (or combined for greater weight) will lengthen carrier dwell time. The cases may/should be relieved from full obturation with the chamber walls and may show less abuse on the case rims where the extractor is holding and pulling.

Paul (LRRPF52) here has shot his 260 AR-10T extensively, and this fella has some loads you might find useful (he seems to think heavier bullets and medium-to-slow powder will help):

http://www.ar10t.com/armalite-ar-10/5772-anyone-have-the-new-ar-10t-in-260?limitstart=0&start=20

 

[Mike_in_FL]

I think the gas port may be too large and the carrier speed is too high. The extractor slips or is torn off so the ejector never does its job. The Tubbs carrier weight might help as was suggested already. You didn't mention what stock or buffer you are using, that could be part of it.

If you search here, you will find threads related to Lilja .260 barrels where the owners were having the same issue with the feed ramp. Lilja corrected this for the owners and it seemed to only be on a few barrels.

I agree with you about the necks, seems like you would have a feeding issue rather than extraction if the neck is too large.

 

[rg1911]

You can try a Tubb Carrier Weight System or an XH buffer -- these individually (or combined for greater weight) will lengthen carrier dwell time. The cases may/should be relieved from full obturation with the chamber walls and may show less abuse on the case rims where the extractor is holding and pulling.

Paul (LRRPF52) here has shot his 260 AR-10T extensively, and this fella has some loads you might find useful (he seems to think heavier bullets and medium-to-slow powder will help):

http://www.ar10t.com/armalite-ar-10/5772-anyone-have-the-new-ar-10t-in-260?limitstart=0&start=20

Thank you. If turning the necks does not completely fix the problem, I'll look into the Tubb system and the XH buffer before opening up the chamber neck.

There's some good information in that link. It looks like I'm already going down the patch of slower powder and heavy bullet.

Richard

 

[sakobob]

Understood. However, I'm old and have had major back surgery that forced me to quit NRA Highpower competition. I can't get into (or out of) position anymore. So I restrict myself to matches that let me shoot from the bench. A trifle depressing, but it beats not shooting at all.

Richard

gotcha, good on you for making it still happen.