AR-15 Cycling Issues with 18" Barrel

[chzhd]

FNG here. I recently put a 18" Proof barrel with a rifle length gas system on my AR-15. I also added an adjustable gas block. The problem I am having is even with the gas block wide open, it will not consistently cycle without the suppressor. I haven't changed the buffer setup from the stock M&P-15 lower and was wondering if I simply needed to swap out the buffer spring. If the spring is all I need to change, what spring do I need to be looking at?

By the way, the ammo I have been running to trouble shoot the cycling has been PMC brand XM193 and Aguilla 62gr 5.56x45. My round count on the barrel is at 290 so far.

It runs beautifully with the can.

Thanks for the help.

T

 

[TimK]

Are you sure the gas block is lined up with the gas port on the barrel? Doesn't take much misalignment to occlude that hole.

 

[Trigger Monkey]

I doubt it's gas block alignment, the hole is usually pretty generous and it would take a noticeable cant to induce issues. I ran into this the first time I tried to run a rifle length gas system on a standard carbine receiver extension, anything but NATO pressure IMI Razor Core wouldn't cycle it properly. I wouldn't mess with the buffer spring but you'll need a heavier buffer like an H2 or H3. Better still, if you have the tools, is to swap it out for a Vltor A5 system. I run BCM's version of the A5 on my SPR with a A5H2 buffer and it cycles everything under the sun.

 

[CaptNemo]

Is the bolt locking back on the empty magazine after the last round is fired when unsuppressed ?
Barrel bore, chamber and gas tube clean ?
Check the simple and free stuff before you start throwing parts at it

 

[MontanaMan]

I wouldn't mess with the buffer spring but you'll need a heavier buffer like an H2 or H3.

Since the OP said he has his adjustable GB wide open, it's probably safe to assume that the failure more is that it's not fully cycling; if that's the case, going to a heavier buffer is going to make the problem worse.

If his GB is not mis-aligned & his GB is not plugged up, he may have a gas leakage or blockage somewhere else or it's conceivable, but not likely, his barrel's gas port is undersized.

All assuming he not shooting really weak-ass ammo in the 1st place.

MM

 

[gigamortis]

I've always heard that the minimum barrel length one should attempt with a rifle gas system is 18.5". You just simply need enough gas dwell impulse between the gas port and the muzzle to get it to work.

 

[TonyTheTiger]

I've always heard that the minimum barrel length one should attempt with a rifle gas system is 18.5".

Maybe ten years ago, 18" rifle gas is the standard these days, 16" rifle gas is doable.

As to the OP, check gas port alignment, both side to side and fore/aft, check gas key for tightness, I once saw a primer anvil end up in the gas key and cause short stroking, check gas rings, measure the gas port and make sure it's not on the small side...

 

[ShooterwithNoName]

Got any more info on the rifle? Are you running a standard bolt carrier group or adjustable? Have you ran this lower before without issues? I would check all the things listed above^^^

I've seen overtightened castle nuts on buffer tubes not allow the BCG to travel all the way back. Actually got a factory assembled lower like that once. BCG was sticking in the buffer tube, after a lot of messing with it I took the castle nut loose and snuggled it back to proper torque, no more problems. It's usually something simple that's easily overloooked.

Good luck with it and if you have any more info on the rifle would help us to diag your issue. It sounds either undergassed or something is either binding, dragging, etc. I would take your buffer and spring out, make sure your buffer hasn't come apart? 18" rifle gas should run just fine.

 

[Onyx Z]

What size is the gas port? If it runs 100% with a suppressor, it is probably on the verge of cycling with the suppressor.

I'm willing to bet the gas port is too small on a barrel like that.

 

[RyanScott]

Gas port should be .093-.104.