The shooter would not have been barricaded in the room for so long if he wasn't able to spray high-velocity rounds at the police. You know those penetrate body armor without ceramic plates and police do not wear ceramic plates.

Revolver rounds would not penetrate, and a carbine would allow him to be flanked much more easily.

EDIT: laws keep honest people in line, but they also give us the ability to punish the dishonest.