New LR308 Build - FTE - Over gassed?

[sulli90]

Well, took my new LR308 build out to the range today to shoot for the first time. Specs are below:

JD Machine Upper & Lower
SI Defense BCG (QPQ coated)
18.5" Criterion Barrel - Chrome Lined - Rifle Length w/ 0.750" gas journal
Centurion Non-Adjustable LP Gas Block (running CMR rail system)
CAR-10 Buffer w/ .308 Carbine Buffer Spring (UBR stock)

In a previous thread I had headspace / bolt jamming issues. This was resolved by swapping out the 7.62 Industries BCG with the SI Defense BCG. The bolt headspaced no problem and everything functions perfectly when cycling by hand (ejection at the 4-5 o'clock position).

So I load 1 round in a 10rd PMAG, fires and ejects no problem, bolt locks back. I continued to do this for about 10 rounds with the same results, but I did start to notice that it looked like some rounds were being ejected forward at the 1-2 o'clock position. I then went ahead and loaded a full 10 rounds. Functioned fine for the first 5 rounds or so and then jammed. The shot case wasn't ejected and was jammed with the bolt trying to chamber a new round. I continued to have this problem intermittently. Sometimes I could go 30 rounds with no problems and other times every couple rounds. I went back to loading 1 round at a time and it ejected fine but some rounds were at the 1-2 o'clock position. Then I had 1 round that apparently did not eject. The bolt locked back, I dropped the mag and the round came falling out with it. I called it a day at this point.

I'm thinking all of this is caused by it being over gassed? Or could it be something else?

If it is over gassed I'm not too sure how to fix it. I don't know of an adjustable gas block that will fit under a CMR. Slash makes a extra heavy CAR10 buffer weight (6.5 ounces vs 5.5 ounces), would that be enough to fix things? Any other options?

Thanks much in advance

[Failure2Stop]

What's your port size?

[BOSAR15]

I had some similar issues with my brand new Maten build. I have an adjustable gas block that I was running fully open, I have an 18" mega barrel, and Shadow Ops Nickel BCG. I do run a CAR-10 buffer and spring.

I did the one round at a time to make sure it was ok, then loaded the PMAGS up. I could fire 5,10 in a row and then I would get the FTE, and the fresh round trying to chamber. I was shooting reloaded 150gr to start, which probably wasnt the best idea. I switched to factory ammo and ran 40 rounds through no problem. I then ran another 110 of the reloaded through with only 1 FTE.

I'm not sure if it was a combo of the ammo, fresh/tight parts or something else. It seems to be running better now. My plan is to slowly close off the gas block to see how low I can go with good function

[sulli90]

What's your port size?

I'm afraid I don't have the tools right now to measure that but I have an email into Criterion as to what the port size should be.

[Failure2Stop]

Really, doesn't matter much since the bolt carrier velocity is pretty clearly high in relation to ejection. Look at the ejector and extractor tension.
Clean and lube the ejector.
What does your extractor spring setup look like?

[RichM]

Sounds similar to a problem I had. The problem went away, so far, after I put an o-ring on the extractor springs. My original setup on the extractor were dual nested springs with no o-ring, the bolt was from Shadow Ops. I think it's a patch for now, I plan on ordering new extra power springs from Sprinco soon.
Hitting the local shop and spending 2 bucks for the o-ring was a quick and easy trouble shooting step for me.

[mic2377]

My 308 had a Mega 18" barrel. It ran like crap at first, ripped rims off, double feeds, destroyed brass, 1 o'clock ejection. Clearly overgassed/under-buffered.

I did the following 3 things:
1. I ended up switching to a A2 rifle length stock/spring/buffer. It is heavier buffer with a different spring rate. The CAR 308 buffer is really too small/light IMO.
2. Adjustable gas block. I would estimate it is 60% open.
3. Removal of the extractor o-ring. The BCG (a CMMG) already had dual springs and the brass was actually hanging up on the extractor instead of ejecting cleanly.

Now it runs 100%, with a nice 3-4 o'clock ejection pattern. Do a little tinkering, you can get it to work. I would try the adjustable gas-block and o-ring first. Another option is to use an A5 buffer tube, AR-10 carbine spring, and H3 buffer, I set up my brother's 308 (also with a criterion barrel) like this and it has never had a single malfunction.