Early CMMG magazines had over sized lugs molded into the magazine bodies that were meant to be filed/sanded down in order to custom fit them to your particular magazine well. It helped to prevent the magazines from rocking back and forth in a receiver with a slightly over sized mag well. A loose fit causes the feed angle to change and causes jams.
CMMG has an advantage with their BHOA "bolt hold open adapter" (optional) that allows the gun to function exactly like a .223/5.56 rifle. The bolt locks open (fully) after the last round is fired. You can drop the empty magazine and the bolt won't close. Insert a full magazine and hit the bolt catch lever to close the bolt. No need to chamber the first round with the charging handle. They also have a forward assist adapter available that allows the forward assist to function normally also.
I have an (awesome) dedicated CMMG upper rather than a conversion, but I also have an original Colt conversion unit that came with a new Colt AR-15 A2 H-BAR that I bought years ago. The conversion kit is still in the original packaging because it's a collectable now. Colt stopped producing them a long time ago.