308 ... I tried the B-square on my Rock River Varmint AR and it worked OK. But, I changed to an ARMS 36EX Extended Swan Sleeve with Warne Rings for more flexibility with scope size, ring placement and eye relief. A friend has the Armalite one piece on his AR-50 and it works great. If you want the one piece mount, the B-Square is nice, but the Armalite is better. Another factor, the B-Sq is only available in 1" and the Armalite in 1" & 30mm. The ARMS Swan Sleeve and your favorite rings give even more options. Good luck ... Mike