It's the Tokyo Marui MTR16 ZET System Gas Blowback Rifle that Stu features in this livestream by Patrol Base... "In this video, Stu sits down with the Tokyo Marui MTR16 ZET System GBB,   based on the M4 Platform, with highly modern furniture such as a Match Twirled Silver Barrel, Ambidextrous and Ergonomic Fire Controls and a Slim Profile Stock."