Don't know the facts on armor piercing, but the news coverage I have seen refers to the police being outgunned. You later see an armored police vehicle tearing down a shack looking for them and police snipers on roofs.
By the way, it was a bunch of cops wearing shorts and vests rolling up in a cruiser with guns blazing out the window that stopped the 2nd shooter (the 1st commited suicide). How could they be outgunned, I'm sure they had a heck of a SWAT team complete with M-16s & MP-5s. Probably the same deal as columbine, the first on the scene were waiting for SWAT but could not contain the subjects. With all those shotguns I can't believe no one got a headshot. Maybe the police were concerned with their backstop and didn't want any more lead flying around.