Of course, what I really want is that quad-mounted .50!
There's an M55 mount I know of available for around ten grand, if it wasn't snapped up at this last KC shootemup fest. Add on 4 Browning .50s [or a pair of 20mm cannons like the Israelis do, or a 7,62 minigun or two like Mike Dillon of Dillon Reloading (ever wonder why he got into making machinery for reloading ammo for mass consumption? Now you know)] or high rate of fire M85 .50 calibers, and you'll be all set.
A pal of mine in Texas has got one, just the thing for use behind an Army *Mule* ATV.