I recently finished my 2A Xanthos build and everything seemed to function perfectly, until I took it down for cleaning after the first range trip.  I detailed the problem and some solutions I tried, but haven't quite nailed down the fix yet.  If anyone has dealt with a similar issue can offer any ideas it would be awesome.  Sorry for the long post, but I wanted to make sure I got all the details included in case it helps with other builds

The BCG was slamming back into the receiver extension threads on the lower.  This was with a 7075 milspec ar15 carbine buffer tube, DPMS 308 carbine spring, and DPMS 308 carbine buffer - The BCG in all examples is the AIM 308 BCG.  The build also has an SLR sentry7 adjustable gas block tuned to barely lock back the BCG on the last round.

Pics of damage:



Once I saw that, checked the clearance with just the buffer and the BCG in the tube, and it was kind of close.  Guessing the bumper was compressing enough to cause contact?



Next I checked out a friends factory DPMS build and noticed his had much more clearance, and no damage:



Then I checked the buffer tube internal depths between the builds.  The DPMS factory commercial tube was shorter than 7", whereas the 7075 milspec tube was slightly over 7"



I read up quite a bit on 308 builds and saw that there were two main recommendations:
1.  Use one of Slash's CAR-10 heavy buffers
2.  Use an Armalite 308 carbine length tube (7.75" internal depth), standard H3 buffer, and an Armalite 308 spring

So, I did both!
Here's a couple pics of the Slash buffer and the BCG checking for clearance, it appears there's much more clearance than with the standard DPMS 308 carbine buffer



And here's the same check with an armalite 308 carbine length tube from POF, and an H3 buffer:



After reading about both combinations I decided to try the Armalite length tube/H3 buffer/Armalite 308 spring since it seemed like it would shoot smoother.  Everything ran well without any malfunctions, but there was new damage!  Not as bad as the first example with the DPMS components, but the BCG is still coming back and hitting the buffer tube and almost the receiver.




At this point I'm a bit concerned.  I have yet to try to the Slash buffer, which appears to be about 1/8" longer than the standard DPMS 308 carbine buffer so that might just be the fix.  However, I would think at least one of the combinations I've tried would have worked without causing damage to the lower.

Any ideas are welcome!

Assemble the rifle. Then pull the charge handle all the way to the rear (past the bolt catch position). Is your bolt face behind (further back) then the rear edge of the ejection port?

Assuming it is, you can drop some quarters down the buffer tube with the spring and buffer out. Reassemble. Do that until the bolt face is slightly forward of the rear of the ejection port.

Thanks for the tip - I just did that and took a couple pics.  I did notice that when the bolt is locked back, it's behind the ejection port - would shimming the tube with quarters to the edge of the ejection port make it so I won't get lock back?

Try it. You should re-post this in the troubleshooting forum. Danno will have pics and lots of info to help you out

It's the extra material at the end of the carrier. I've been noticing this lately, something i didn't catch before.

I don't take pictures so don't have photo evidence to back up this claim.

There is variation in the rain lengths machined into these carriers, as well as extra material at the step down for the portion that goes into the buffer tube. Carrier tail i guess? It's not alot, were talking a few thousandths here, maybe more. But enough to be noticed with the baked eye when comparing multiple carriers with different brandings.

I believe this is where all this lower damage is coming from, combined with the cheap loosely spec'd 308 carbine buffers. This is the cause if everything else is within spec, meaning threading of the receiver boss, and lower receiver dimensions.

I'll see if the local shops still have a decent selection and get some pictures. My mental photos don't transfer well.