Expanding on what Leo said, criminals, terrorists, etc, always have been, and always will, be able to get what ever arms they need. Countries, including our own, will supply terrorists if it suits the needs of that government. Criminals will steal guns or purchase them on the black market, which is thriving and will grow, and become more powerful, as legal firearms and ammunition, are restricted or outlawed.
Governments, all of them, create the atmosphere for crime to flourish. The more government restricts freedom, imposes regulation, the higher the crime rate. There are only two ways for them to stop it, one would be to have a system like in Singapore where you can be caned for chewing gum, or the better way, kill off the black market by opening up a legal, less regulated, source of what the people want.
Don't believe that government causes crime to escalate by regulating what they have no business regulating? Just look at Prohibition, the "Drug War" and cigarette smuggling, just to name a few.