I am entertaining buying a new ACR. I never owned one in the past but I remember seeing threads in here and else where that discussed barrel lugs being peened by the bolt. Is this a common problem or something that is still happening?

I don’t know if it’s common or still happening, but I believe the reason why is due to “loose design” or loose tolerances. For example, the AR15 spec for the barrel extension pin slot is .126 -0/+.002. I measured 3 ACR trunnions and they were as much as .135! That's .007 outside of the tolerance and, possibly combined with other sloppy tolerances, such as the carrier, cam pin, rails, or receiver, may be why some bolt lugs and barrel extension lugs show chipping due to misalignment.

Is this a problem or is it not supposed to happen at all? I really like the rifle  but not sure I want to get something that has an issue that can cause premature failure of a very crucial part.

It happens but isn’t an issue.  It happens within 5 rounds and doesn’t change after it wears in.  I have barrels with over 10k on them and they all have it.  It’s because of the extension pin cutout in the trunnion like CJ said.

I have never seen any issues on any Acr associated with the peening on the barrel extensions.  It’s just an initial wear point on the platform.

Just adding in that its not an issue, and as others said, due to the trunnion design.

The peening that happens (if it happens) isn't going to cause any structural issues.

It actually is pretty common in many firearms - glock slides do it, AKs do it, some AR15s do it.

Good to know. Thanks for the information. I think I will pick one up next paycheck.