I think most understand the 308 "AR-10'ish" guns are not as standardized as 5.56 and other small frame.

So it's good to stay with the same vendor, or at least "pattern". Even with that you can find sneaky issues. The DPMS "LR308" is one of the most common ones, and is the defacto standard for clones. (IE: Not KAC, LMT, etc).

I ran across one, and thought I'd share.

TL;DR Human error is more common than machining errors

Situation: New AeroPrecision LR308 complete Lower and Complete Upper; LR308 pattern BCG known to work with the LR308 and specifically, AP.

All looks good, except the bolt catch does not catch on the bolt face. The BCG cannot go back far enough for it to do so, by 1/2". Otherwise functions correctly. (shoots good, even)

The immediate thoughts are 'I bought a dog', "should have bought an AP BCG", etc.

But some troubleshooting led to a completely different direction.

- 1st suspect- Bad Bolt catch or AR-15 catch. Visual inspection confirmed it was a 308 catch, and the bcg would not go back anywhere near far enough for it to be a machining issue.

- 2nd suspect- Short Buffer tube. Nope, the LR308 patterns use an M4 standard buffer tube. Which was confirmed. Which leads to...

- 3rd (and common) suspect- Buffer too long. LR308 patterns use a shorter buffer than AR-15. (2.5" vs regular 3.5" M4 carbine buffer). Common error. But, nope, this one was a correct 308 2.5" buffer. This should be the 1st thing to check.

So the immediate suspicion is the BCG... "should have got from the same mfg". But wait a minute, toolcraft 308 BCG's are pretty well respected, and this one look right. Specifically, it does not appear to be 1/2" off.

- 4th Suspect (and again, common)- Spring is too long, wrong # coils, etc. So the test was removing the spring, reinsert the buffer, and there is plenty of room for the catch to catch the bolt face. So the spring was limiting the rearward travel.

Some research led me to learn that LR308 springs are different from Armalite AR-10 springs are different from M4 223 pattern springs. (Length, and coils).

And specifically, the spring in the 308 carbine I was working with was too long and had way too many coils. Thus was fully compressing and stopping the BCG sooner than it should.

A quick call to AP, they fedex'd a correct 308 spring. Which addressed the issue.

Not sure how it happened. Did they grab an M4 spring from the wrong bin at the factory? Did the store swap it out to address a problem, then replace it with the wrong one? Don't know.

AP made it right, and it addressed the issue very quickly. AP was excellent to work with, even though I'm not sure it was their issue.

So it was not mis-machined bolt catch, or BCG. Nor too short buffer tube, etc. Very simply wrong spring. Simple problems, not complex.