PTT:
What I'd like to know is the justification for having such arms. Why do you need anything more than a revolver or a single-shot rifle or shotgun available to everyone?
Paraphrased from Massad Ayoob, a highly thought of firearms expert here in the Good Ole US of A:-
Initially the police carried .38 Specials to deal with crime and criminals and civilians who chose to be armed carried the same. In the 70's and 80's police changed to semi-automatic pistols and shotguns. The armed public followed suit. In more recent times the police have added an AR-15 type "assault weapon" with high capacity, (30 round), magazines and the public have again followed suit.
The reason is simple. The police are the experts at protecting the public and, as such, have determined that the weapons they require to do so are a semi-automatic pistol, a shotgun and an AR-15 with a high capacity magazine. It is only logical therefore that, in the period where the public await the arrival of the police, the best weapons for them to protect themselves with are those the police would be bringing.