Hogwash. There are no laws that can prevent criminals from being armed with firearms. Even nations with confiscatory gun laws still have a few crimes committed with firearms.
When defenders have firearms, criminals face a risk of injury or death. Maybe big risk, maybe small risk, but a greater risk than they face if the victims does not have a firearm. Sooner or later, the criminal will get shot, even if he is armed. Criminals know this, so they fear facing armed victims, no matter what they are armed with.