Rifled Foster-type slugs from a semiauto 12ga with some sort of inexpensive folding backup irons, either Magpuls or a knockoff, mounted atop it. Notice the smoke from the muzzle brake and the not-yet-ejected spent shell still traveling rearward visible in the ejection port.
Between the brake, the gas operation, and the inline nature of the stock, the recoil is driving the gun straight to the rear with virtually zero muzzle climb. However, the inertia is also causing the rear folding BUIS to flop rearwards.
