I'm in agreement with a bunch of folks. BAR's, submachineguns of a variety of types, 22LR buzzguns, silencers, shorty shotguns, and maybe we'd be seeing repeating handguns in calibers over .50. And I want a 1917 MG, they have a pretty slow cycle rate.
Ideally, I'd like a nice artillery piece. The 8" 'long tom' howitzer is still in the armed forces inventory, and was one the most acurrate guns of WW-II and Korea. (I don't suppose you can run on of those by yourself, though.
) A 'match-grade' 105mm would be a neat competition gun. Bullseye at 12,000 yards, anyone? For historical fun, I think a Wehrmacht '88' would be like owning a Luger. And for true select-fire over-the-topitude, a Bofors 40mm AA gun would really be the cat's meow.
Recoil-less rifles have a muzzle velocity of of 1000 fps or so typically, regardless of bore size. For those days when you can't make it out to the 'long' artillery range, the lightweight recoil-less rifle could pack in the car trunk, and with the right ammo and backstop, you could recycle projectiles. 25mm wadcutters at say, 500 yards? Wonder what kinda groups you could get?
Ah, to dream...