This is a repost of my ar15.com forum thread, but I think you guys might give me a more straight up answer. I've tried researching this for the past couple of days and couldn't find anything. Maybe you guys can give me some insight. My Carbon15 does not have a buffer retainer in the lower receiver. I have to remove the buffer tube to breakdown my ar15 because the buffer rests against the bolt carrier. My lower receiver cracked and I purchased a RRA lower (still waiting to pick it up). When I looked at it at the store, I saw that the buffer is retained in the buffer tube like it is in all other ARs that I've seen. Now my bolt carrier does not have a groove cut out in it for the buffer retainer like the three bolt carriers pictured below have. (left part of the picture, horizontal groove) Here a picture of my bolt carrier, buffer tube threads, and buffer itself. The carrier bottomed out on the buffer tube and expanded the very front of it which caused the top of the ring on the lower receiver to crack. I tried repairing it with some epoxy seeing as it's just carbon, but that failed to hold. It cracked as soon as I started threading in the buffer tube. I assume I'm going to need a new bolt carrier also, is there any other problems I will come across besides the carrier?