Good morning,

I have a 16 inch CMMG .308 and I took it out with the new glass I got for it for a sight in. First round, the round ejected at about 4-5 o clock and the bolt slammed forward and didn't feed another round. I cycled a new round by hand and then the bolt got stuck on the edge of the upcoming round, again ejecting at the 4-5 o clock. I tore the rifle down at the range and cleaned and lubed it. No difference. I continued to hand cycle at least 40 rounds of several different ammo selections I had. I checked the gas block, no leaks, aligned, and set properly. I did find out this rifle has a non adjustable gas block.

My question is, is there a way to adjust the gas without an adjustable gas block? For specifics, I have the Midwest industries low profile gas block.  My presumption is I'm a tad under gassed. I had this rifle built for me by a local ffl who is no longer with us.

Will a lighter buffer help me? Or should I just install a new adjustable gas block? It is a rifle length system.

I have a base understanding of how the system operates, I just don't want to go the wrong route when there could be an easier alternative, cheaper too.

Thank you!

Did you try a different mag?

Yeah, I have two 20 rd pmags and one 10 rd. Tried all 3, all same result... unless it's the pmags

A lighter buffer tube will not help you in any way
If your buffer is 3 oz, the only other option would be to take weight off of it
If you are under gassed a new gas block will not help you, because most likely your gas block has a bigger opening than your Barrel
You could try a semi auto BCG if you are using a full auto
There has been times under extreme conditions that I have had to open up the gas port on the barrel and then adjust with adjustable gas block
Trying different loads can sometimes help, are you using Factory or are you reloading?

Darn. And i see what i did there. And it's a semi auto. I used two different factory .308 hunting loads and surplus 7.62x51

I'm not saying it's time to open up the barrel gas port... But if you do, do it in .002 increments and put a cleaning rod in the barrel so the drill bit doesn't touch the other side of the barrel
There's always the buffer spring to consider...

I don't believe I have the equipment to perform that precise of material removal... however I know a guy.

I guess I'll have to experiment with some things and see of I can solve the issue.

Equipment needed is a pair of calipers, cordless drill, drill bit

It does not lock back, and I could fire more but I did around 50 rounds, maybe more, with no improvement.

Clean it real good, run it wet and use mil spec ammo to break it in . If after 100 rounds it is still giving you trouble call CMMG and they will get you taken care of . If it is a factory CMMG rifle it should be 100% reliable . I would hold off drilling the gas port larger , that may over gas the rifle and beat it up . Check out the feed ramps and see how they look with an empty mag in . Will it lock back if you push up on the  mag ????

Use a wood dowel and the stops on the drill (or put masking tape on the drill bit) so you don't waste a cleaning rod.

It locks back by hand like it is supposed to with an empty mag. The mag does not impede the travel of the bolt. The feed ramps line up and look to be correct as well. I see they're wearing slightly different than my ar-15 though. A lot of brass transfer is on the teeth of the ramps and some in the groove. If that makes any sense. On my ar-15 the bullet seems to follow the ramps perfectly. The 308 hand cycles fine otherwise.

EDIT. I pressed down on the mag with moderate force and I cannot bring the bolt back sometimes. I tried the same thing with my ar-15 and it functions fine every time. During hand cycling, my bolt did not go all the way forward one time and stopped mid stroke with a loose round in front of it. I'm beginning to lose my mind.

Sounds like it could be a magazine issue, I would try a different manufactures mag (not poly).
It also sounds like it could be an under gas issue.  I would check the alignment of the gas block.  If the holes are not aligned, it would explain why it wont feed, and not lock back on the last round.

AS

Was this a factory built cmmg? 308 ARs seem the be especially finicky when you build them. I had issues like you describe and the only thing that I finally changed to fix the issue was replace the Amazon special gas tube with a spikes and it runs whatever I feed it.

Especially on a new build, only load one round in the mag and fire it for the first few shots. The bolt should lock back. It may not lock back on the first round, but shoot 10 or 7 rounds while you are breaking in the barrel. Load one round at a time until the bolt locks back with the mag in. If this doesn't happen with good ammo, start looking at gas block and tube alignment. Take the bolt out of the carrier and check that the rings are properly oriented.

Also, make sure you are running a true 308 buffer; not an AR15 buffer. Especially in a carbine length tube.

Good place to start anyway