Nah, I got a much better idea
If you own a military grade weapon in America, you CAN keep them, but it would be a major financial liability. If you use your high-powered firearm in a mass shooting, and not for self defense, not only your firearm will be taken away and dismantled/destroyed in front of your face, but you will lose all of your assets (house, car, bank account) on top of a life sentence behind bars. All of your wealth will be taken away if you do a mass shooting killing innocent people. That would make people think twice before firing it.
Regular handguns are not subjected to these rules.
How many mass shooters do you know of that are free?

after they let them babies get slaughtered in sandy hook and nothing was done, that was it.
If someone uses a rifle in a mass shooting they either end up dead from the police or suicide before getting arrested or they spend the rest of their life in prison for the crime they committed.