Like His Newtness, I think big.
76mm HEAT M4 Sherman main battery (with tank as gun platform). The 76mm was a big improvement over the original 75mm - much more so than the trivial extra millimeter would suggest. The Sherman is still simple enough to be (vaguely) practical for private ownership. I'm guessing that this would be classified by the ATF as a "Destructive Device", so the cost will be higher because of the extensive legal work to get all the licenses.
Ma-Deuce (P-51 as the gun platform). Because what's more awesome than Ma-Deuce?
Six of them riding on the greatest propeller fighter of all time. These planes are popular at air shows all over the land, but this would take that to a whole 'nother level. Not just low passes, but
strafing runs. Like with the Sherman, I expect that you'd need to budget extra for legal fees (or
bribes political contributions) to get the right licenses. Hey, it's my fantasy, right?
Mark 8 Torpedo tube (PT Boat as gun platform). Yeah, it's a wooden boat, and so upkeep will be a pain (buy stock in Valspar), but let's face it - if you want to play one ups-manship at the local marina, this would about do it (note to self: make sure that proper grey camouflage paint job is done before taking it into harbor). New England residents will get bonus points if you paint a number "109" on the bow. And if you dock it in Hyannisport.
Siege of Constantinople bombard. There's a reason that the city is called Istanbul, and this is it. The old Theodosian Walls were no match to the firepower of these monsters. Yeah, it took a team of oxen to haul it and you were lucky to get off three shots in a day, but those old Roman fortifications just couldn't take the pounding. Bonus points: this is likely not covered by the NFA (worst case, it's classified as a Curio and Relic firearm). Downside: local meth-addled thugs will want to swipe it for the scrap value.
A 2.7 mm (10 caliber) Kolibri, simply because it's about at the opposite end of the size spectrum from the Sultan Mehmet bombard. The whole pistol isn't much bigger than a single .45 ACP cartridge (shown in the picture for scale), so this is a mouse gun that might only make the mouse mad if you hit him. Actually, I couldn't think of anything else, but this is goofy enough to add to the list.
I'm thinking I could score this sweet list for $20M or so - $30M tops. Might have to look at one of the former Soviet Bloc countries for an old bombard, but I understood that the rules stated that
money is no object. I expect that this is the strangest list posted in this meme, but you guys know that any incoming transmission from Planet Borepatch is going to be weird. It was actually fun asking myself what will dial the weird up to eleven. Sure, it's really not what Robb was getting at in his meme, but like Newt, I pride myself on being visionary. And unlike a Moon Colony, this isn't idiotic.
Well, it is, but it's not
that idiotic.