What happens when the Stinger levels up?

Secondary weapons don't have camos, and their max level is 10, not 31 (this is for primary weapons only, with which you in fact get a golden camo when you reach max level).

The only thing you can get with using the stinger is titles.

Edited for the correct answer : The Stinger's max level is 10, and as @rdurand says, you only get titles. (Sorry about the mistake, havent been playing in a long time :P)

The good thing about the Stinger is that it has a faster Lock-On than the Javelin, and usually most air streaks get downed in 1 shot with the Stinger (unless they have flares, or are hugeass copters/planes).

Stingers also have more ammo than Javelins.

When the Stinger levels up, it simply levels up and gives you bonus XP, unless you quit in the middle of a match.