Originally Posted By: yotedog65284The indexing coller is what has the feed ramps cut in it. I know bushmaster and CMMG use them.
Actually, the barrel extension is either an "M-4" extension or it's an "AR-15" extension. They both have feed-ramps made into the barrel extension. The M-4 feed-ramps are just cut at a steeper angle, and go deeper.
The receivers are a different story. They either have M-4 feed-ramps cut into them, or they don't. It won't cause you any problems to use an M-4 barrel extension on a receiver that does not have the M-4 ramps cut into it, BUT, if you have a receiver with the M-4 ramps cut into it, you will be unhappy if your barrel extension is not cut with the M-4 ramps. If you had a situation like this, the tip of the bullet would, or at least could, run up the ramp on the receiver, and then would hit the lip on the barrel extension. This would at the least damage the tip of the bullet. More likely it would cause you function problems.
I haven't seen an AR-15 barrel extension around for some time. Everything that I have in the shop is M-4. I think most manufacturers just dropped the other extension and only use the M-4 ones now.