I agree. Every gun starts out legal. The challenge is keeping the initial gun owner from selling or giving it to a criminal or a crazy person. Registration and tracking, with mandatory background checks and criminal and financial penalties for people who sell their guns without doing the registration and backgrounding, would make legal gun owners think a lot harder about selling their piece to a criminal or their crazy uncle Frank with the gun fetish.
I also think that requiring insurance would help a lot. Insurance requirements would make gun owners and gun manufacturers internalize, to some extent, the external costs that gun ownership is putting on society, and add incentives for responsible gun ownership. If you proved you had a safe for your gun, your insurance would be lower. If you proved you were trained, your insurance would be lower. If you buy a .38 revolver, your insurance will be lower than if you buy an AR-15. Have a buyback program so people can get rid of their guns if they want when the new laws go into effect.