Situation: the "rewards" for leveling up a primary weapon are the accessories (silencer, straight pull etc.). These stop somewhere between level 21 and 29 (depending on weapon type).
Complication: you can advance a gun until level 100. This seems rather pointless, if after level 21-29 you get nothing in return.
Proposed solution: add a special skin for maxing out a gun - awarded either when you reach level 100 of that gun or when you reach it & you pay a certain amount of gp/chips.