300 Blackout will not cycle

[roysha]

I put together this AR 300 Blackout from all new parts. It has the carbine gas tube and 16" barrel. It will absolutely not cycle.

This is what I have done so far trying to figure it out:

I have used 4227 and H110 and in both instances increased charge till I was .5 grain over recommended max. Primers were looking a bit iffy so I stopped. I have used the LEE 155 cast and HOR 168 jacketed bullets.

Bought a box of factory ammo. No joy! Huge waste of money and time.

I opened the barrel gas port to the suggested max. of .125"

I have checked and rechecked and rechecked the gas block to port alignment since it seems to be a gas flow issue. Installed a gas block and tube from one of my other carbines that I know is very reliable. No difference. Gas tube and gas block have no obstructions.

I took the lower off and tried it on a different upper. Ran fine.

Took the 300 BO upper and put on a known reliable lower, won't cycle.

Compared buffer and buffer spring to other carbine buffers and springs and as near as I can tell they are identical.

Even went so far as to install a piston conversion to see if that would help. Nope.

I have read that the AR needs to run wet. Believe me it couldn't have been any wetter.

Polished the chamber lightly with 320 grit cloth in case there was a bit of roughness.

Slugged the bore. For all practical purposes, spot on .308.

I know that the pistol length gas tube is the most flexible but I also know there is a BUNCH of carbine length guns out there that are running fine.
Before I set the barrel up and turn it back so I can install a pistol gas length tube I would like to hear from you folks if there is something I do to make this cycle.

[Elkins45]

Wonder if the upper itself has an alignment issue? Can you try a different BCG from a known working gun? That would tell you if the problem is in the BCG or the upper.

Did you check the gas rings on your bolt? You may be leaking out all your gas. Check and make sure the gaps aren't lined up and that they are snug.

[Huntsman52]

Did not see above in you post but have you tested the bolt carrier group in another upper? Maybe an issue at the gas key? Do you get carbon fouling on the bolt?
As for the piston, I have read but do not have experience that the 300 BO and a piston system can have issues. Thought I would point that out.

Hope you get it fixed.

[milrifle]

I bought my AR new 30 years ago, but when I first got it, it would not cycle. Someone told me it needed to be broken in. I took the bolt/bolt carrier assembly out and sat in front of the TV and pushed the bolt in and 'slung' it back out over and over and over for an hour or so. Next time I went to fire it, it worked fine. Just like an engine, my rings needed to seat.

[xacex]

Are your gas rings good? If you can stand the BCG up on the bolt without it sliding down they are good, but if the carrier slides down to the closed position the rings are bad. Try swapping out the BCG for another rifle. Gas key leakage could also be causing this. Most of the time you will get some movement with both of these problems, but not complete cycling.

[cmdrted]

another gas block related suggestion is check that the gas port hole in the barrel and gas block line up.

[Gtek]

Yep, a lot of the barrels I have played with the block needed to be pulled forward from machined shoulder. The barrel shoulder is cut for using a forward hand guard retainer and pushing all the way back, cats eye. I double check mine with a friends cost way too much bore scope.

[wordsmith]

I own and have built several 16" carbine gas system 300 BLK's, and they ran perfectly with H110 loads. I would agree with others that you most likely have a gas leakage problem, either at the gas block or in the BCG. On one build, I had similar gas issues (short stroking) and was pulling my hair out until I noticed the gas block set screws had backed out slightly and allowed gas to escape around the edges. I retightened using Loktite and voila, problem solved. There is a very good chance you have a gas leakage problem, and I suggest focusing on the gas block first.