This stubby device is the X Products can launcher for the AR15, one of several options offered. Other variants work with AR10, and a .22 blank insert converts other 37mm launchers to the same functionality. It works like a rifle grenade launcher: a pop can is loaded from the muzzle, then a blank cartridge is loaded into the barrel-less chamber, and the gun elevated to launch the can out past a hundred yards.
Due to the considerable internal volume of the bloop tube, the report is very mild. Accuracy is good enough to hit a car at the extreme range. The purpose? Plain fun, a marksmanship challenge, and an alternative to clays for shotgunners.
Safe backstop and access to NFA guns permitting, the pair above is using a select-fire Saiga 12 with buckshot to shoot down cans in flight. A hit with a single buckshot pellet changes the trajectory of the can, with soda jetting out of the hole. While the smallest birdshot can’t penetrate the aluminum at long range, hunting loads in the #4 to #6 range work well in the more conventional, semi-auto or pump shotguns. Thanks to the shiny bottoms, cans are highly visible in flight and usually buck the wind well. With generic sodas selling for about 20 cents, the targets aren’t prohibitively expensive, and add a nice variety to wingshooting practice.