Paintball makes lots more sense. You can get the guts for a repeating action at Wal Mart.

Here's the online selection - costs less than $200.

They shoot 300fps -- dangerous, but not lethal. They can make a huge red or yellow splat. They are decently accurate over 30 yards or so -- and will shoot close to 90 or so - maybe more. Ammo is equally available. You can see the projectile in flight, yet it is fast and large enough to be impressive. Finally, with appropriate eyewear, you can fight 'em nondestructively against other, like-armed tanks, and clean up with water. You can even go up against the other radio controlled tanker.

You could even mount a lipstick - sized video cam on the tank and require use of the cam only for warfare.

Make the opposing tankers fair targets, too. That's an attack on headquarters.