it's for breaching, as mentioned before - the end is usually knurled or jagged to give it a grip on the door, and it has massive gas slots/ports so that the shot gasses go someplace nice instead of out the breach of the shotgun into the shooters face.
it's usually used with a sintered frangible slug, or small shot. the idea is to destroy the door hinges or lock/knob assembly so the door just falls off or opens - without causing serious injury on the the other side of the door.
of course standard antipersonnel ammo can be fired through it too.
Ua Mau ke Ea o ka ʻĀina i ka Pono