Ok, I just rec'd my new BCM BCG and I'm trying to decide what to do with it.  I had only really needed just the BC itself, but since BCM isn't filling orders on those, I had to go whole-hog.

I have the following in need of upgrade:
- Bushmaster upper w/ complete Bushmaster AR15 BCG
- Colt 6721 upper w/ complete SP1 BCG

Should I swap out the BM AR BCG for the BCM BCG, 1-for-1?  Then I could sell the complete BM BCG to help fund a Colt M16 BC for my 6721.

Are there any concerns with putting a new bolt into a rifle with a used extension?  It has less than 3k rounds through it, IIRC.

I've never heard of a barrel extension failing (not that it can't or has not, I've just never heard of it).

I say you're good to go with the new bolt.

I have put new barrels on with old bolts and never had a problem.  With wear the parts will only make more space for the round than what they would be brand new. In your case if you get a new bolt/carrier they will only be closer to what it was new, but not quite because you have wear on the barrel extension. So your head space is not quite the same as it would be from the factory but is should be closer than what it was with two worn parts. Not that the two worn parts together are even close to worn enough to cause problems.

Thanks for the input . . .

This BCM BCG is incredible!  I think it it quite possible the most beautiful set of AR15 parts I have ever seen.  I almost feel as though I'll be soiling it by installing it in a Bushmaster upper.