criminals don't go into gun shops to buy guns.

If there were fewer guns available from the outset, like in Canada and the rest of the western world, then the time required to get a gun goes up dramatically. The more time that a nutcase requires to get a gun, the more likely it is that he a. gives up, b. uses a knife instead, c. makes his intentions known which can lead to stopping him, etc.

If I suddenly went crazy and wanted to go to a mall or a movie theatre and shoot everyone there, I would have no idea where to get a gun except the police department or the army base. So, I'd have to start hanging around the less desirable parts of town and start making enquiries as to how to obtain an illegal weapon (most of which are imported from the US). That's an awful lot of work and would take a long, long time before I'd be able to do it. So, I'd have to have gone crazy but managed to be schizophrenic and appear to be living my normal life with normal habits all the while arranging to get a gun and ammo. And, not just any gun, but something that can shoot multiple rounds quickly. Had I lived in the US, I would assume that if I broke into 3 houses, I'd have already found at least one gun.

So, while there are a number of good suggestions mentioned elsewhere, like limiting the size of magazines, it seems as though banning handguns (not used by hunters) would be another good one and anything else to limit the ease with which criminals can obtain guns and that means having fewer guns total.

Actually, if I wanted to try to find a gun in Canada, I'd google my nearest hunting lodge and try to break in there or follow someone who frequents the lodge and break into his place. All still time-consuming and increasing the likelihood of getting caught before I made it to the mall, school, movie theatre, work place, etc.