That depends very much on the country. In places like Japan or Singapore, it would be next to impossible for an average Joe to acquire an illegal firearm, unless they're willing to smuggle it in from overseas themselves.

I guess. To be honest this is mostly a cultural trait of compliance. The laws didn't stop Shinzo Abe's killer from making a homemade gun from pipes. Guns aren't really that fancy. It's 13th century technology and fairly trivial to construct.