7.62x39 11.5" AR Problem

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wally

wally

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This might be better in the Gunsmithing and Repairs forum but I suspect most of the SBR expertise is here.

I've an M&A Parts 11.5" 7.62x39 AR upper and it appears to be short stroking badly. Suppressed it seems to run fine, ran a mag and a half this morning after trying everything I could think of unsuppressed. I've tried standard, H, H2, and 9mm buffers, a couple of different recoil springs and nothing seems to let it lock the slide back when single loading one round in the mag. Suppressed it always locks back. With two in the mag it fails to strip the last round, sometimes the empty ejects, other times not. Single rounds always eject, but weakly.


I even swapped in the MGI bolt & carrier from my 16" 7.62x39 carbine upper and it made no difference either :)

I see no evidence of any gas leaking around the FSB.

I assume the next step is to open up the gas port in the barrel. My question is what should the port diameter be on an 11.5" 7.62x39 AR?

Any other potential solutions I've possibly over looked?

Accuracy seems very good and the POA/POI shift suppressed vs. un-suppressed is pretty negligible at 50 yards using a red dot optic shooting off sandbags.

My main motivation is to run it suppressed, but I'd feel a lot better with working un-suppressed, as I doubt it'll handle much fouling the way it is now.
 
Hmmm, 100 views and no feedback as to if I'm on the right or wrong track. Could the moderator move this to the Gunsmithing and Repairs forum?

My port seems to be ~0.062 as an 0.059 drill slips in easily and an 0.063 goes part way in.

Would drilling it to the next size up, 0.067 be a good first attempt, or should it be even larger for an 11.5" 7.62x39 AR?

No evidence of any significant gas leakage (barely noticeable carbon on the tube when removed) around the front sight base or gas tube, no FOD or kinks in the tube.
 
As I said in the initial post, I've tried: Standard, H, H2, & 9mm. Not a lot of difference. I ran it suppressed with the Standard buffer after the three heavier buffers failed to solve the issue.
 
I would say gas; have you had someone watch you shoot the rifle and watch the BCG?
If it is undergased or do not have a good seal for the gas tube and have a leak then that to could cause a problem. We had another guy that had to enlarge his gas port due to short stroking. Sorry could not be more specific but if gas tube is unblocked and you have a good seal then...... I would first call the company who sold the upper and ask them what size and how big you can go and see what they say and go from there.
 
Its a standard front sight and base. As I said, no sign of any leakage around the front sight base or the gas tube, the tube is clear and without kinks.

After the first time out, I squirted a good flow brake parts cleaner down the gas tube and it all came out the barrel suggesting the port and sight base are aligned. I had the 9mm buffer in the SBR lower and didn't bring any of the others or another AR that first time out with it, at that point I was expecting a lighter buffer to be the solution.

Actually the bolt, carrier, and charging handle seem unusually clean for having fired ~60 rounds through it. Using Brown Bear ammo, it runs fine (from the same box) in my 16" 7.62x39 AR so I blame the gun not the ammo.

I figure I'll open the port to about 10% more area and see what happens.

Going from the existing 0.062" diameter to the next drill size 0.067" is about a 17% increase in port area. If I go too far I guess I can always buy an adjustable gas block.
 
My guess is that the dwell is wrong, since it works properly with a can on it. But opening the port to .067 shouldn't be too much.

Fortunately, the barrel isn't chrome lined so you don't have to worry about damaging the bore lining. :)
 
What's the distance from the gas port to the muzzle?
Click to expand...

I should have mentioned this, it appears to be the same location as for a 16" carbine.

Googling gas port sizes suggests 0.062" is "standard" for a 5.56 carbine.


If you go too far you can get one of those fat gas tubes to lower pressure.
Click to expand...

Thanks I didn't know about this option. Who sells them? as I've never heard of them before now.
 
I can't remember for the life of me who makes the things sorry. I know there's a company that makes a pig tail one that has some coils in it to make extra volume too.
 
his prob sounds like it's not enough gas.suppressor adds pressure,and it works,call the maker and get the correct gas hole diameter for the round.mark the gas tube where it enters the gas block.remove tube and see if the hole in tube matches up with gas block hole.
 
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I'm thinking you need to be more about .083-.087 but I'd reach that in at least two incremental steps with a perceived common load. Much beyond that and you can begin to shear jacketing material especially with a poorly addressed port. I like to drill first and reach the finish diameter on large ports with a reamer. I've gone .093-.095 in special applications without undo effects but I had eased into those numbers by degree just in case.
 
Going from 0.062 to 0.067 wasn't quite enough. Single round locked back, loading two both fired and appeared to lock back, but actually the bolt was hung on the follower and slammed forward when I removed the mags. Firing from a fully loaded magazine closed on an empty chamber. All this was with a standard buffer.

At least going in the right direction. I'll open it up to 0.070 and see how it goes. This will be a 27% larger opening than the 0.062 original.
 
followup:

I'm thinking you need to be more about .083-.087 but I'd reach that in at least two incremental steps
Click to expand...

Pretty much right on the money!

I'm at 0.082" (in three steps) and got feed from a full 30 round mag and always locking back on the last shot using the standard carbine buffer. Ejection was about 4-5 o'clock, so I didn't try any heavier buffers.

Had a few failures to feed with different mags (~15-20 rounds remaining) where the nose of the bullet was in the chamber, the bolt on the cartridge base but not cleared the mag feed lips, and not far enough forward for the assist to do anything, a little pullback (~1/4") of bolt with the charging handle and release chambered the rounds.

I think I'll open it up another ~).005 and try an H buffer (or more) to have a little more momentum moving the bolt forward and get the ejection closer to 2-3 o'clock.
 
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Yipee! Like the man said: "easy does it".

Be sure to clean the bolt and carrier interior well between test dimensions as low pressures can some times leave residue/deposits which might otherwise have been safely blown through. If the deposits are left till a port dimension that offers good cycling is found, and then the deposits are removed - it can sometimes present by then as (and in fact could be) an oversized port.

A simpler way to express it is.. "Crapola, I've over drilled my port to overcome a nastified BCG.

Rare, but I've seen it a couple of times.
 
Happy ending!

Opening it up another 0.005" from 0.082" to 0.087" got me perfect function with an H2 buffer both suppressed and un-suppressed. I didn't try any other buffers as I liked the way it cycled and shot.

Ran two mags unsuppressed, two suppressed, and another suppressed (using five different mags). Ejection un-suppressed was was about 4-5 o'clock and all locked back on the shot. No gas in the face suppressed either.

Would have shot more but the Ulyanovsk was having a 10-15% failure to fire despite having a nice primer dent. The Wolf and Brown Bear gave no issues.

I was ready to blame the ammo since none of the duds would go a second time in the SBR, but since they all went when fired in my 7.62x39 AR 16" Carbine with the MGI bolt, I figure I need to increase the firing pin protrusion a bit, or order another MGI bolt. Oh well, I need another project :)

This thing is a joy to shoot, and shooting steel plates at 75-85 yards I didn't change my aim point a bit between suppressed and un-suppressed with the Vortex Strikefire sight after my initial 50 yard zero.
 
I am just getting into this type of firearm (AR) and the approach to find/resolve problem is what I find most interesting. Thanks for sharing this.
 
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