Gearbox is mostly the shape actually.
V1's are for the very first guns that came out (FAMAS)
V2 is for M4/16 and variants
V3 is for AK and variants
V6 is P90 IIRC
V7 is M14 IIRC
Although there are modifications made to the design to make it better it's mostly the shape actually.
ಠ_ಠLess QQ more Pew Pew
READY TO >> RACE