Okay, Katie, I'll bite. Those criminals that don't pay the gun tax, because they steal their guns, will indeed shirk the tax. However, where did those guns start out? They weren't manufactured in some dark warehouse somewhere with "Illegal Guns" stamped on the side of the building. So they came out of a legal owner's possession or from a legally run gun shop that was robbed.
What is difficult about the concept that any measure that reduces the amount of legal guns in an areas will necessarily reduce the amount of illegal guns in the area (other variables aside)? I'm not saying we should do away with gun ownership wholesale, but I don't think it's unreasonable to allow cities with huge amounts of gun violence to take measures to reduce it.