As an Department of the Army civilian ordnance technician in the early 1980s, then switching over to the Navy, one of the projects I was involved in was an effort to develop 20 and 25mm gun mounts for the M3/M6 37mm main guns of the M3 and M5 light tanks and the M8 armored car, still very much in use in South and Central America and elsewhere. The 37mm high explosive round supply was was running very short and in the little 1.5-inch diameter 37mm, there wasn't much of a charge anyway; about like a hand grenade. So prototype mounts and feed mechanisms for the tank guns were rigged for 20mm and 25mm automatic cannons, really more useful for crowd control and putting down palace insurrections anyway.
And then the bright idea hit: why not rework the 37mm to take the 30x173mm round of the A10 Warthog's GAU-8 rotary-barrel cannon. It was tried and tested, first in a single-shot fixed testbed gun, and to their horror, Army testing found out it would zip right through the armor of an M113 personnel carrier. Or an M2/M3 Bradley-both sides, front to back, whatever. Or most anything else they tried it against, including several friendly foreign vehicles, save the front armor of an Abrams tank and I don't think they wanted to know the answer to that one.
Even better: the WWII 57mm M1A1 antitank gun known as *the six-pounder* to the Brits.
By the way, the Brazilians reworked some of their old M3/M5 chassis, and they too still have some small military value, though now less desirable to collectors. But even the X1 and its cousins are now museum pieces.
The X1A1 was the tank version, fitrted with a nothing-to-sneeze-at 90mm main gun. And their upgrades to the WWII American M41 tank have brought it right into the XXI Century as well:
