Define that please. What makes something a "weapon of war"? What makes a gun a "military grade" weapon?
Military grade rifle: The M4 carbine. It can fire "fully-automatic." You squeeze the trigger and keep holding it down and it continues spitting out rounds until your magazine is empty. The AR-15 (ARmalite-15) is a civilian semi-automatic rifle and currently the most popular rifle purchased and used on rifle ranges.
The cheapest weapon to rack up mass casualties: It's sitting in your driveway or, if you don't have one of those four-wheeled killing beasts, you can rent one or even more cheaply, steal one. Then just pick the sidewalk of your choice in the city, town, or wide path of your choice. Obviously, the more people walking on them the better to rack up points with. The terrorist in Nice, France racked up a total of 85 points (kills) on his short drive. We've had a few terrorist attacks with motor vehicles, but so far, their tally isn't as impressive as the one in France. They will no doubt work out the kinks in that, eventually.
So, for mass killing, cars and trucks are your weapon of choice and stealing them is the preferred method. It doesn't cost you anything, the public won't ban them since they all use them and don't want to see anything that they personally use, taken away from them, and they're easily found.
So, forget about military or non-military style weapons. Nut jobs are going to go after us with whatever they can grab (knives, machetes, hammers, screwdrivers, chainsaws, axes, et cetera).