Under gassed AR, what can i do?

[greyling22]

Before I did anything crazy I'd swap a couple parts between your 2 guns too see if you can isolate the issue. Swap bolts. And I would never suggest swapping uppers for a couple rounds at an empty range or something to see if the issue was in the lower because that would be super illegal.


I didn't see it mentioned, but i probably overlooked it, But are you sure the gas block is installed correctly? Centered over the gas port both side to side and front to back? In my opinion that's the most likely culprit if you have a low profile gas block.

 

[jdh]

My favorite: trim one turn at a time off of the buffer spring.

(Don’t do this.)

 

[Demi-human]

(Don’t do this.)

And then try to stretch it back out when the bolt won’t close.

 

[greyling22]

My favorite: trim one turn at a time off of the buffer spring.

(Don’t do this.)

or you could try it before poo-poo-ing on the idea. I had a blackout SBR that wouldn't cycle subs whilst I waited on my 30 cal can. After taking off 2 coils it fixed the problem. Now, I wouldn't take off 6 coils, but a spring is what, $5? It's a cheap experiment.

 

[psyopspec]

Alternatively, you could suppress it and see if that helps.

 

[MistWolf]

...you could suppress it...

Best idea yet.

 

[jdh]

or you could try it before poo-poo-ing on the idea. I had a blackout SBR that wouldn't cycle subs whilst I waited on my 30 cal can. After taking off 2 coils it fixed the problem. Now, I wouldn't take off 6 coils, but a spring is what, $5? It's a cheap experiment.

Back when I was moonlighting at the gun shop I ordered a bunch of replacement springs for those who followed that advice. It may work sometimes but it is still a Bubba fix. Like the guy who wants to lower his truck by cutting coils of the springs instead of buying a lowering kit then wonders why it handles like crap all of a sudden.

 

[BreechFace]

..

 

[12Bravo20]

Good sharp drill bits fed at the correct rate will not leave a burr on the interior especially if you are simply opening up an existing gas port a few thousands of an inch. I have done it many times with a drill bit and accuracy has not suffered. And even worst case and you do leave a minor burr 100rds later the burr is gone. If you got the reamer use it, it certainly will not hurt anything but if all you have is a number index of drill bits I would not go out and buy a reamer for fixing one barrel. Use good lubrication and a good vice on a drill press or milling machine and you will be just fine.

As a retired machinist/tool and die maker I will state the key words here: Proper lubricant (cutting oil), Proper speed for the bit, Proper feed of the bit into the material. It is the same for both drill bits or reamers.

And I have to agree, that if you have everything setup correctly with the proper speeds, feeds, and lubricants a drill bit works just fine. No need to ream unless you really want to and have reamers on hand. I would not go out and buy reamers just for this type of job, especially if it is only 1 or 2 barrels. Good drill bits are cheaper than a quality reamer. And no matter which you use, spend the money for good quality.

 

[Hjalkarsson]

I would have to agree that you should check all the other possibilities first before you ream the gas port. should be a last resort. there are SO many overgassed uppers, and pistol uppers are the worst for overgassing, it is extremely highly unlikely that you have an undergassed gas port. I'd put that in the "winning lottery ticket" category.

out of those other dozen possible causes, IF it turns out to be the buffer weight/spring, check back with us before proceeding. it's complicated to do it right. you have to balance the buffer weight, BCG weight, and spring power, to equal the gas pressure you're getting, and that itself results in a balance between BCG travel to the rear, and bolt speed. getting it wrong results in accelerated wear if not broken parts in the near future.

 

[someguy2800]

The first thing I would do is look for some place where the gas is leaking out. If there is a big leak at the gas block or gas key you will see streaks of fouling. If no leaks are apparent the next step I would take is to take apart the buffer and dump out the weights. If this causes it to lock back then you know for certain it is under gassed and you can decide what do from there.

 

[Hjalkarsson]

let's summarize the possible causes.

1. excessive fouling/dirt/sand. (needs cleaned)
2. some internal part loose and dragging, slowing down the action.
3. buffer too heavy and/or spring too stiff.
4. weak ammo.
5. worn gas rings.
6. new gas rings. not broken in yet.
7. loose gas block
8. loose carrier key,
9. gas tube moves back and forth due missing gas tube roll pin.
10. hole/crack in gas tube.
11. gas block not lined up with gas port.
12. gas tube clogged with debris or fouling
13. gas port too small.

 

[Cokeman]

Has it ever worked? Did it start out with this problem or is this a new development?

 

[RevolvingGarbage]

Another cheaper and less involved idea would be to use a muzzle brake with some kind of expansion chamber, similar to what short AK’s will typically have. I’m not sure which are available in the proper thread config for an AR, but this should increase back pressure enough for you to make sure that’s the problem. As suggested above, a suppressor would also serve the same function although obviously that is a more expensive proposition.

 

[TrickyDick]

Just start with trying different ammo, then go from there...

 

[brasscollector]

Centered over the gas port both side to side and front to back?

11. gas block not lined up with gas port.

If I was a betting man this is where I'd put my money down. A barrel with a gas block journal that is machined for a handguard cap to be placed between the barrel and gas block may not align correctly with a low profile GB and no handguard cap. I had this happen to me once, simply moving the GB down the barrel the thickness of the cap fixed the issue for me.

 

[MistWolf]

There is no mystery to the AR gas system. Start with 5.56 ammo and the buffers are always H, H2, A5H2 and Rifle. (You could fit an H3 buffer on that list, but it would be better to switch to an A5 or Rifle to get a longer spring.)

The only variable is barrel & gas system length and suppressor use to determine proper gas port diameter.

For example, a 14.5 or 16 inch barrel with a carbine gas system (no suppressor) uses a .0625" diameter gas port. Use an H, H2, A5H2 or Rifle buffer and standard spring, an AR in this configuration will be within it's correct span of operation every time. (If it doesn't, the problem is elsewhere.)