I've always seen flat tops referred to as A3, with the exception of RRA and their A4's.  I've also heard some complaining about RRA flat tops not fitting certain mounts and the like.  Can someone clear this up for me?  Thanks-

Unless mistaken, the military refferes to flat tops as A4's but the commercial market, for the most part, reffers to flat tops as A3's. So there is a little confusion about that. Some companies like armalite use the "A4" military term to describe their flat tops, while others like bushmaster use the more commercial market "A3" term. I dont know why this is so, I just know it is so.

Roughly a year ago, Rock River put out a batch of flat top uppers where the rails were just a little too large. If you buy a new production upper, you should not have this problem.