Criminal get guns in some specific ways: They sleal them from a known mark, They Buy them at licence or unlicence store They smuggle them pass borders, Etc With good regulation you can prevent a some of these occurrence and then reduce the number of gun in circulation. For example, forcing every gun owner to have is gun in a locked gun storage cabinet would reduce theft. Forcing a background check for all gun store would prevent known offender to easily purchase guns. As the number of easily available gun decrease, the price of the easy gun increase. You get criminal who will carry less and start preferring blades and blunt weapons instead or no weapon at all because getting them will be easier. And hopefully, you reduce gun related death. It's not magic, it's not total and it's not impossible criminal will still get guns just less of them. And other countries have succeed.