Oh yeah, GIJoe had more than its fair share of giant weapons.
The SAW-Viper's rifle, the Rock-Viper's sniper rifle, v1 Shockwave's MAC-10 and handgun, the 90s Gung-Ho's machine gun, Tunnel Rat's M-60, the new MP5K that the last VvV Duke and the DTC Range-Vipers had, Spearhead's rifle, 91 Dusty's submachine gun... the list goes on and on.
Barrel Roll's sniper rifle from DTC, it's based on this design:
http://world.guns.ru/sniper/sn46-e.htm and from what I've seen of it, it looks to be in close scale with the real deal.