Ok i know about a .300WM ,,but i thought i seen on here some one talking about a 300blk or some thing like that .
I think if im going to build one ,i would like to start out with the 5.56
.300 blk is a 30 caliber bullet that uses a trimmed down 223/556 case. This allows shooters to use the same AR parts (lower, mag, Bolt carrier group, upper) except for the barrel. Right now it is expensive but the price should come down at some time. That's all I know for now.