He also has another leather strap with push buttons that attach to a metal plate on the right side of his waist (back) that comes through a hole in the cape and holds the rifle in place when its on his back, in addition to the actual strap the rifle is attached to the bandolier with
And also his knife, which would probably count as a prop anyway, although it mostly resides in his right shin-guard!
(First Picture shows the metal plate with leather strap, and butt-plate, as well as how the backplate seems to attach with velcro etc.)