Looks like a Bushmaster A3, fitted with a GG&G *MAD*auxiliary rear sight instead of the usual *loophandle* rear iron sight, and an EO tech holographis electro-optical sight unit.
It's what might be thought of as the thinking man's setup, and for one of two reasons, though having a Stradivarius does not make one a concert violinist, a person who has their equipment so arranged so as to reflect their tastes and augment the weapon to best suit their chosen techniques is likely a formidible ally or a dangerous opponent.
Oh, and he's got pretty interesting taste in reading material, too.
A local dealer should be able to set you up with the rifle once you know what to ask for, and you can add the rest yourself. The book you can get directly from the author, just by clicking the link.
-archy-/-