I think if private security, and privatized military contractors, and other such private entities are allowed legal access to the arms and armor of their choice, than so should the rest of civilian population.
If you want to stop people having access to unnecessary weapons, then perhaps weapons production should also not be in private hands relatively free to sell those to anyone anywhere in the world.
What about the rest of the world that will not stop producing and selling weapons even if the US does stop.
Lastly, whats to stop people from filling the niche of senseless violence with home made variety of weapons and have lot more explosives instead of guns in the streets.
Long story short, you cant stop violence, you can only remove the need for violence by building happy and healthy society and even that not 100% successfully.