Where should you fire assault rifles? Who should own machine guns?
Face it, folks love to shoot guns, especially powerful ones! We can't change that. So, we let them. Let them (adults only, you nitwits) go to Orlando, or Arizona, or to Tuscaloosa, around the corner, or wherever. When they are there, let them fire whatever gun the owner can let them use under federal license.
Let's don't be pikers, let them throw grenades, shoot bazookas, or run a box of ammo thru an old MG-42. Let them shoot hell out of all the zombie or alien simulations they want. No people targets, please--way bad example.
In fact, theme parks are the only place anyone should have access to assault-style weapons or weapons with high capacity magazines. Let's treat military style weapons and their accessories the way they deserve to be treated.
They should be treated like we treated dangerous animals in zoos in earlier eras. They are dangerous, so we make sure they stay confined and don't get out of their cages and into public spaces where they might hurt people.