Who is the manufacturer of the BCG?
I'm thinking possibly a rough BCG. An upper being out of spec can also do that. It's mostly the upper that determines the angle the bolt carrier is going to go back at. I've seen some so out of spec that the buffer will get stuck an inch or two into the tube. I would try a different bolt carrier and then a different upper if you can and see where the problem continues. If it is the upper there's not much you can do but replace it.
The bolt rear is causing that or the buffer is bottoming out and slingshotting back HARD causing the Buffer Detent to gouge it.
Use red lipsock on the buffer and fire one round. See if it hits the inside of the rear of the buffer tube. if it does use a heavier buffer or spring till it stops.
If the rear of the carrier is burred it needs to be reprofiled and polished.
The bolt rear is causing that or the buffer is bottoming out and slingshotting back HARD causing the Buffer Detent to gouge it.
Use red lipsock on the buffer and fire one round. See if it hits the inside of the rear of the buffer tube. if it does use a heavier buffer or spring till it stops.
If the rear of the carrier is burred it needs to be reprofiled and polished.
I'll add that if all that doesn't work then about the only thing left to do is to have Chad do his .223 to .40 S/W conversion on your rifle. It's one of his specialties.
Maybe this will help...
http://www.weaponevolution.com/forum/showthread.php?1744-BOLT-CARRIER-LAPPING-DEBURRING-PROCEDURE
-Swift