don't do it, you don't need it, the only time you need a SB die is if you have an undersized chamber, the AR-10 is chambered 7.62x51, it is a generously cut 308 chamber. the only rifles I ever saw that needed them were browning auto's.
use regular dies, and keep track of how much spring back you develop, then either bump the shoulders a bit more or anneal the brass.
small base dies resize the brass to just under minimal specs, if you don't need them you wear your brass out in 1/2 the time.
RR