If you want to make and fight an RC Tank, check out this site:
http://www.rctankcombat.com/ It's still a relatively new sport, but it looks good so far. One guy is converting a GI Joe Sherman tank for combat.
The main gun of the M4 Sherman series was originally a 75mm, aside for some artillery and tank killer versions. That works out to about a 3 inch bore for the main gun. A quarter of that would be right in the vicinity of a 12 or 10-gauge shotgun. It'd need to maintain an 18-inch barrel length to remain legal, or can be built and federally licensed as a *short-barrelled shotgun* with no stock following federal licencing application as such and payment of a $5.00 fee. Alternatively, a 27mm signal flare pistol might be used.
Accordingly, I believe I'd think Quarter-scale rather than 1/5th if I were you. And I'd note that model railroaders have come up with equipment suitable for adult human ridership/occupancy with rairload equipment reduced to a quarter of it's prototype's size.
But oh yes, it can certainly be done.
-archy-/-