Saying nothing for or against banning guns or ammo in XYZ city.... Your logic doesn't follow. Lets do a little thought experiment.Banning the legal sale of guns in a city will do nothing to stop guns from coming into the city, when you could buy them literally anywhere else in the province.
Imagine a city where there are no bike shops withing 20 miles of you, and you can't buy bikes or bike parts online without having them sent to a bike shop to have a bunch of paperwork completed. Do you think more or less people would have bikes compared to a normal city?
Making something harder to get (more of a pain in the ass) will reduce the amount of that thing in circulation, assuming stable demand.