Any DPMS compatible mag will work, I'm really happy with MidwestPX's shortened Pmags. The 7 rounder is awesome for prone shooting and hunting, it barely sticks out of the mag well, and is 100% reliable.
I personally prefer the really short ones that basically sit flush with the magwell of my P-308, but you can get them in a little longer length still limited to 5-rounds if you prefer.