AR gas question

[LanceS4803]

Hopefully we have some knowledgeable AR smiths here.
Have built multiple ARs, from SBRs to AR-10s, absolutely no issues. Until now.
I stick with Aero, and have used this receiver, bolt carrier and buffer/spring combo before. So, known parts.
This rifle has a 16" Hanson profile barrel, mid-length gas system. Will fire, reset trigger, but bolt doesn't go back far enough to pick up the next round and fails the bolt hold open test.
Using same buffer and springs as other builds with same length barrel/gas system.
Bolt moves freely, gas key is tight with good engagement of tube, all rings in place with great seal.
Pulled the low profile gas block (screws and roll pin), holes lined up very well. (I set all my gas blocks with an SLR jig.) Checked barrel, no obstructions or burrs in port hole. I don't have the exact pin or bit, but hole is slightly larger than .070. Air flows through block and tube.
The only thing I didn't do yet was swap lowers to try and isolate an upper issue from a lower issue. Too hot, sweaty and just forgot.
Anyway, I've reached the end of TI'ing this thing and open to ideas.

 

[tug]

There are only a few things that make your gun run. Gas and spring tension and weight, ammo.

What is your ammo? .223 or NATO.

What is your ejection pattern? My guess is it is at 5 o'clock.

Is your gas block wide open? Is the gas tube pinned in the block. Is the gas key installed correctly.

If all of this is good, then check your buffer weight. Too heavy? Or is the spring too stiff?

Before you do anything, take that upper and put it on a different lower with the same ammo.

Take a known upper and put it on the lower with the same ammo.

A lot can go wrong, but it should not take more than 30 minutes to run it to ground.

 

[backyardtndeer]

Before you do anything, take that upper and put it on a different lower with the same ammo.

This is where I would start, to identify where the problem is. Then try another mid length upper with the same ammo. I have had ammo I loaded that would run carbine length and shorter fine, but would not properly cycle rifle.

If you identify the problem to be with the upper, try a different bcg, easier to swap then to take the gas block and tube off. If problem persists, then it just about has to be a misalignment issue or something with the gas block.

 

[Big John in TN]

Had the same problem with a 7.62 X 39 I built. Problem was the bolt binding in the carrier. Was going to send it back but lubed it real good and put it back together. Runs fine now. Hope this helps.

 

[infoman jr.]

I assume the front face of the buffer has no marks on it? if it's H1 or H2, you should try the lighter carbine buffer which is unmarked.

 

[rifle02]

I can't tell for sure from your picture of the gas port but it almost looks as though the drill bit did not break through completely. I have had that issue before.
As an absolutely Last Resort when all else fails I have enlarged the gas port 1 drill bit size.
Of course this is not done with a hand drill. And it's good idea to have a cleaning Rod or something else in the bore to make sure you don't hit the other side when you break through.

 

[LanceS4803]

I'll be test firing this weekend, and bringing a couple of known ARs to swap parts with. Should isolate the problem pretty quick.
Just frustrating that the 3rd build using identical parts from front to back (except barrel profile, same brand BA, on this one), would give me a problem.

 

[Omega]

Definitely sounds like a gas issue, not enough force to push the BCG far enough back. If the gas system seems to be well aligned, and the tube is clean, I would default to looking into the spring/buffer which may be too heavy for your gas system.