Calling all AR experts (particularly AR10)...

[berettaprofessor]

I took out the new Stag LH AR10 (that I described building in a recent thread) to see how it functioned and to get the scope on paper at short range.

Upside; didn't have to touch the scope adjustment (see pic). First shot at 15 yards was on a 1" stickie dot, and 7 others kneeling weren't far off at 20 yards, so I decided to wait till I can get to a 100 yard range. If I can shoot it kneeling like this, all while wondering if the first few shots will blow me up, I'm excited to see what it will do when it's benched at 100.

Downside: it's a single shot. Chambered fine loading from magazine every time, and it seemed to extract and eject perfectly (empties about 4 feet back and left 3 feet from the LH ejection port and no marks on the brass), but every "auto-self-load" jammed the second round between the chamber and bolt. So I have some questions:

1) In my limited experience, I would guess that the bolt is cycling too fast, moving back forward before the round can load, and the rifle is overgassed. Is that likely correct?

2) I'm guessing that shooting it a bit more to "give it a chance to loosen up" won't help?

3) I used a low-profile gas block and rifle length gas tube on an 18" barrel. I know that in the previous thread, BFOOSH006 suggested using an adjustable block, but I already had this one ordered at that time (sorry bfoosh006). Should just I bite the bullet, order an adjustable block, and swap it out?

4) If it's not the gas block, what else can I try? I was only using one magazine, but it was a new Magpul AR10, 10 round mag, so maybe a different mag? I only loaded a maximum of 2 rounds at a time. The buffer tube/buffer/spring is a standard AR10 set ordered from Stag and I can't find any other buffer weights specifically for AR10 builds. I did have a LimbSaver pad on the butt; can that be interfering with function? The ammo was cheap Federal Power-Shok 150 grain.

 

[pdsmith505]

While I don't own an AR-10 for a direct comparison, I did pull out both a fixed-stock (A2) AR-15 and a carbine AR-15. As you can see, the rifle buffer and spring are significantly longer than the carbine buffer and spring. Placing a rifle buffer/spring combo into the carbine will not allow the bolt to move backward far enough to chamber a round, let alone cycle.


The below photo shows an AR-15 rifle buffer (top) and an AR-10 rifle buffer (bottom). Note that the AR-10 buffer is shorter to accommodate the longer bolt/stroke of the AR-10.


My understanding is the rifle-length buffer tubes themselves are compatible between AR-15's and AR-10's, and based on that, I'd imagine that you would run into the same issues trying to run a rifle AR-10 buffer/spring in an AR-10 with a carbine buffer tube. Below are pictures from Stag's site of rifle and carbine setups for AR-10's:




From your other thread:

Based partly on desire, partly on stupidity, and elated by several successful AR15 lower builds, I've wanted an AR10 for years and since the complete rifles seem a bit pricey compared to AR15's, I decided to build one. Recently, Stag announced they were releasing an AR10 upper LH receiver based on DPMS compatibility (yes, I like LH AR's, but that's another discussion), so I jumped on it and ordered one, figuring I'd buy a decent lower later and complete it. The saga:

1. July: Hmmmm...in reading further, I found out that Stag's AR10 uppers have a unique angled fit, different than DPMS upper rounded patterns, so it would best if I order a Stag lower and do it soon while they're available. And look, Stag just emailed me a sale on Stag AR10 lowers and LPK's. Sold!

2. Look, there's an email with a sale on Stag's 308 buffer tube and buffer. Sold! Added a PMAG adjustable stock.

...

I'm going to say that you don't have a rifle length buffer in your AR-10 if it is able to chamber and eject a round.

Probably over-gassed. You don't have to buy a gas block from Stag. Figure out what diameter your barrel is at the gas block (likely .750") and purchase an adjustable block. My understanding is that AR-15/Ar-10 gas blocks are interchangable... gas tube length may vary depending on what pattern Stag used, but you can re-use the one you have.

 

[berettaprofessor]

Well, it surely chambers by hand. And, you're correct, I just checked and what I ordered was the carbine buffer and spring. So the proper one is in there and I'd better start looking for an adjustable gas block. Stag doesn't have one. Any suggestions?...the block on there is 0.750". I was thinking of this one...https://www.midwayusa.com/product/1...le-bleed-off-set-screw-gas-block-ar-15-lr-308

 

[FL-NC]

Are you sure that your gas block is properly indexed? Is the BCG lubed VERY well?

 

[TIMC]

Looks like a gas block issue to me. Had very similar issues with a recent AR10 build. I ended up changing to another gas block and drilling the flash hole out. No further issues.

 

[MistWolf]

The 308 AR usually uses an A5 pattern RE. That requires the use of a carbine buffer and a rifle spring.

Before spending money on an adjustable gas block, check the ejector & spring and the extractor & spring. The problem your rifle is having is much like what happens when the extractor spring is weak and loses control of the cartridge rim.

If your rifle was so over gassed that the BCG was running faster than the magazine could feed it, the recoil would noticeably sharp, almost painful, especially with a 308.

 

[ColoradoMinuteMan]

No offense to previous posts, but I think it's short stroking. The BCG is moving far enough back to extract, but not cycling far enough back for the bolt face catch the case rim of the next cartridge in the magazine. The locking lugs are instead scraping along the top side of the case and partially stripping the cartridge from the magazine. You can test for short stroking by loading a single cartridge in the magazine. Load and fire the cartridge. If the bolt does not lock back after extraction, the BCG is not cycling back far enough to allow the magazine follower to rise and activate the bolt hold open.

A short stroke can be caused by all sorts of things but in a new build I'd be immediately skeptical of the gas system (most likely not enough gas). Since non-adjustable gas blocks are usually the equivalent of a fully opened adjustable gas block, I'd be surprised if an adjustable gas block would automatically fix this issue. I'd start with the easy stuff and move on the harder stuff. Take a look at your gas-key an ensure it is secure, take a look at gas-key to gas tube alignment. Take a look at your gas rings and ensure your BCG is lubricated. Next thing make sure that your gas tube is properly pinned into your gas block. Look for any obvious gas leaks around the block as well as it being properly aligned to your gas port. If you find nothing obvious you can pull your gas block and ensure ensure your gas port on your barrel is free of any burrs or blockage, then make sure your gas block allows air to pass through it and is free of burrs. Realign and put it all back together and try the bold hold open test again.

Give it a try and get back to us.

 

[MachIVshooter]

Since it's ejecting properly, it probably isn't short stroking, but run the check mentioned above to verify.

I've had issues with my PA-10s running suppressed, had to make the gas systems adjustable. Easy on the FF handguard one. On the other, I drilled & tapped the FSB, made a stainless 10-32 slotted thumb screw for it:

 

[wbbh]

I have an AR10 that is short stroking as ColoradoMinuteMan describes. Most shots it like a single shot, but about three out of ten shots it double feeds. It does this when shooting some (factory) reloaded ammo that is supposed to be NATO spec. I suspect it is underpowered, when I shoot some Hornady 168 grain projectile rounds function is 100%.

For my next trip to the range I'll try some other comer loads as well as some actual surplus NATO ammunition.

 

[MistWolf]

I should have suggested a lock back check. That's the first step in trouble shooting an AR.

 

[berettaprofessor]

I think it was locking open, but I’ll run the check tomorrow and report back. I’m pretty sure the block was properly indexed and the tube was open. Gas key is secure and it mates with the tube well. Gas tube is pinned. I’ll have to look at the gas rings again and make sure they are properly positioned.

 

[ColoradoMinuteMan]

Since it's ejecting properly, it probably isn't short stroking, but run the check mentioned above to verify.

I've had issues with my PA-10s running suppressed, had to make the gas systems adjustable. Easy on the FF handguard one. On the other, I drilled & tapped the FSB, made a stainless 10-32 slotted thumb screw for it:

View attachment 767415

Very cool custom made adjustable gasblock. It very well may not be short stroking, but it sure looks like it to me. You can easily extract a casing without the carrier going all the way back. Since the ejector is always under tension, the case can eject as soon as it clears the receiver. In fact, I just simulated it with an empty casing at low speed by slowly pulling back the charging handle and the moment the case clears the receiver it ejects and the case ejected even though I only pulled the BCG back about 2/3 of a full cycle.

I'm interested to hear how the bolt hold open experiment goes. I hope you have good luck finding and fixing the issue.

 

[243winxb]

If bolt locking back, then a magazine problem?

Load more then 2 rounds. The more the mag spring is compressed , the faster the rounds will move up.

The mag has to let the case head (round) move high enough in the mag for the bolt to pick it up and load.

When my new (1979) M16 came from Colt , it would not feed at all.
The 20 round factory magazines were the problem. Bent the metal mag lips to allow the rounds to sit higher. Fixed it.

 

[HankC]

I also think it is magazine related with a combination of fast cycling. Some AR10 magazine's front wall needs to be lowered by removing some material; otherwise, it may cause round to rise early before bullet tip hitting the feed ramp. Feed ramp should be what guides the bullets into the chamber. While magazine feed lips hold the round in the magazine, the base of the round will have dive since the tip rises too early riding on the front wall and cause the bolt ride over the round and jam. 3-point jam. When hand cycling, there is enough time to allow the round fully rise and pass the front lip, in firing at high cycling rate, there may not be enough time for the round to fully rise and cause the jam. Some magazines will need to lower the front wall. My Midway USA AR Stoner AR10 mag (made by ASC) is like that. Take a look of your magazine with couple rounds in the mag, the bullet tip should be significantly above the front wall, my AR Stoner mag was just a little bit above! Sort of remember I had 3-point Jam before, I filed the front lip down, maybe 1/16", and now perfect! It won't hurt anything if it is not the cause but some assurance!

 

[bfoosh006]

I should have suggested a lock back check. That's the first step in trouble shooting an AR.

This... first make sure the BCG locks back.

If it does, then you are most likely over-gassed and the BCG is cycling to fast.

Your choice for an adjustable gas block is fine.

Can you post a photo of your current buffer and spring ?

There are "heavier" ones available... but we need to be sure of what you have.

I also suggest the Tubbs .308 Flatwire recoil spring ... it has added spring weight when the bolt is closed on the breach... where you want it... without the extra weight on the locked back position.

 

[bfoosh006]

As for heavy .308 carbine buffers...

https://www.kakindustry.com/lr-308-parts/308-lower-parts/lr308-carbine-buffer-heavy

How many rounds have you actually fired ?
I ask because .308 AR's seem to have a little more of a frustration break in period.

All 3 barrels I have broken in did.

A little tinkering and polishing various places made a ton of difference.

You all can see my AR308 fixes over here....

https://forum.308ar.com/topic/15124-pa10-nitrated-18-nb-bcg-gen-2-observations/

 

[berettaprofessor]

UPDATE: Thanks to all who are replying and trying to help. Much appreciated!

I took apart the BCG this morning. Gas rings are staggered as necessary, some slight resistance to them as bolt seats in carrier. Then I fired it using a different magazine (still a Magpul), and without the Limbsaver pad. Three rounds loaded single in magazine, two different ammos (federal 223 and Winchester 7.62), bolt locked back each and every time on empty mag. Then tried a couple of doubles again. Same result as above, extraction complete but 2nd round jammed against feed ramp.

Some other observations to answer other questions:

I looked at the ammo in the mag and it sits low in relation to the front wall but so do my AR15 pmags. Both AR10 mags that I have look the same. I went out this morning to buy something besides a Magpul mag; nothing open on Sunday except Academy and all they have for AR10 are magpul. I’ll pick up something steel this week somewhere.

Bfoosh, I’ll post a pic of the buffer and spring in a separate post today. I’ve fired exactly 14 rounds through it so far.

Brass all looks good after firing, no pressure issues visible.

Recoil is stiff but doesn’t seem excessive, however I’m not very recoil sensitive with rifles (I’m not into magnums but have shot M1s, Mausers, and Mosins before). I haven’t in the past shot much that isn’t 243 or 223 so I don’t have a reference for how 308 is normally. Today, it was still comfortable without the
Limbsaver pad.

I had the bolt disassembled to check headspace recently and unless I could screw up the extractor spring like putting it in backward I think it’s okay. Certainly stiff enough that I can’t move the extractor with my fingernail alone and it securely holds a nonfired round when extracting from chamber manually.

I have some Hornady 168 and will try it sometime this week if I get off work in daylight.

Any other thoughts are appreciated. I have an adjustable gas block on the way and will put it on if this doesn’t resolve some other way before I get around to it.

If I (we) can get this problem solved, I’m really liking this rifle otherwise.

 

[berettaprofessor]

Bfoosh, attached a picture of the buffer and spring. The buffer weighs 3.9 oz. I'm now seriously wondering if that KAK heavy buffer you suggested might work.

 

[bfoosh006]

UPDATE: Thanks to all who are replying and trying to help. Much appreciated!

I took apart the BCG this morning. Gas rings are staggered as necessary, some slight resistance to them as bolt seats in carrier. Then I fired it using a different magazine (still a Magpul), and without the Limbsaver pad. Three rounds loaded single in magazine, two different ammos (federal 223 and Winchester 7.62), bolt locked back each and every time on empty mag. Then tried a couple of doubles again. Same result as above, extraction complete but 2nd round jammed against feed ramp.

Some other observations to answer other questions:

I looked at the ammo in the mag and it sits low in relation to the front wall but so do my AR15 pmags. Both AR10 mags that I have look the same. I went out this morning to buy something besides a Magpul mag; nothing open on Sunday except Academy and all they have for AR10 are magpul. I’ll pick up something steel this week somewhere.

Bfoosh, I’ll post a pic of the buffer and spring in a separate post today. I’ve fired exactly 14 rounds through it so far.

Brass all looks good after firing, no pressure issues visible.

Recoil is stiff but doesn’t seem excessive, however I’m not very recoil sensitive with rifles (I’m not into magnums but have shot M1s, Mausers, and Mosins before). I haven’t in the past shot much that isn’t 243 or 223 so I don’t have a reference for how 308 is normally. Today, it was still comfortable without the
Limbsaver pad.

I had the bolt disassembled to check headspace recently and unless I could screw up the extractor spring like putting it in backward I think it’s okay. Certainly stiff enough that I can’t move the extractor with my fingernail alone and it securely holds a nonfired round when extracting from chamber manually.

I have some Hornady 168 and will try it sometime this week if I get off work in daylight.

Any other thoughts are appreciated. I have an adjustable gas block on the way and will put it on if this doesn’t resolve some other way before I get around to it.

If I (we) can get this problem solved, I’m really liking this rifle otherwise.

Don't worry about the gas rings being staggered.

It is over gassed... the .308 round has AMPLE gas volume...there is a great chance it is out running your mag spring.... the buffer pad might even be bouncing off the bottom of the buffer tube.

Buy and install the adjustable gas block.... leave the Limbsaver pad on. ( it has no effect on the rifle cycling )

You will be shocked how much nicer the rifle shoots.... and will soon be singing my same swan song.

Want another testimony ?

https://forum.308ar.com/topic/15535-pa10-shooting-good/

It will be fixed.

 

[ColoradoMinuteMan]

The adjustable gas block is quite nice to have on a 308 AR. I have an SLR Sentry adjustable gas block on mine and it certainly makes a noticeable difference in BCG impulse when dialed down appropriately. As mentioned above, the .308 produces plenty of excess gas. I hope this resolves your issue.

 

[M1key]

Don't own an AR10, but AR rings don't have to be staggered and the AR bolt should meet with resistance when inserted. I'm thinking that the heavier bullet may cycle better.

M

 

[HankC]

If it is new, send it back to Stag to get it fixed under warranty. Also check the mag when inserted to see if the front wall is at or below feed ramp. If higher, grind it down, let the feed ramp guide the feeding.

 

[M1key]

edit

 

[bfoosh006]

IMHO... the heavier KAK buffer only help a little. When I tried it... it really wasn't enough.

Buy the adjustable gas block. Buy once , cry once.

 

[MistWolf]

There is no evidence the problem is over gassing.