I don't remember where I read this, but one of the Founders wrote something about the 2nd amendment referring to the common arms carried by the military. In other words, the private citizen can own whatever the government would issue to a combat troop. Large caliber high capacity machine guns, sniper rifles, handguns, silencers, grenade & rocket launchers, etc. If they can carry it, so can we.That would obviously exclude tanks, nukes, ICBMs, cruise missiles, large bombs, etc.
I suppose one could own an armored personnel carrier, aircraft carrier, jet fighter, B-52, or nuclear sub as long as they are converted for commercial, research or recreational purposes.