Not to be rude, but officers utilize a myriad of weapons such as rifles, handguns, shotguns, and tasers. This is a shotgun forum and the thread is about shotguns so try to stick on point.
I will say that the AR-15 is the go to weapon for certain tasks, but for some it is the shotgun. Often an officer with one will work in a team with an officer with the other.
The fact is that law enforcement's overwhelming choice of shotguns is the 12 gauge.