The "thing" under the cop's rifle (or should I say deadly horrendous evil baby-killing assault weapon) is probably a bean-bag shooter. It's a "less than lethal" weapon. Shoot an unruly suspect in the chest with a high velocity bean-bag, and he might reconsider being cooperative.
AB
Thanks!
I thought maybe it was one of those 12-guage pump shotguns that mounts under the barrel, but it looked too slender.
I hadn't considered that it was there to make for a potentially less deadly weapon.