I thought Dlew's suggestion about using only rubber bullets was interesting, but I'm not sure if that is a good idea. I think that could potentially increase gun-related fatalities in the US.
I just think that most people, even criminals or law-abiding people protecting their property, really do not want to take a life and will only use their gun as a last resort. If their gun is loaded up with rubber bullets, though, I don't think they would even hesitate to use it. Instead of a guy walking into a gas station and demanding that the clerk give him all the money out of the register, he might just shoot the clerk with a rubber bullet and then take the money when he's incapacitated. If somebody steals your wallet and you are packing a gun with rubber bullets, you probably would just shoot the guy in the back when he's running away, because he will probably survive, and you would like to have your wallet back.
I just think that legislation would create a shoot-first mentality, and while rubber bullets are far less fatal, the increased use would at the very least increase gun-related injuries, and might even increase deaths. They would make mass shootings far less tragic, though.