Ammo Belt:
-Takes shield slot
-Reduces movement speed reduction when firing
-Increases clip size
-Reduces reload time

Why it's balanced:
It allows players to put a greater focus on gunplay and kiting without all the advantages of a shield. If a gunslinger gets stuck in a crowd, they won't have an easy way out like a shield-using gunslinger or a sword user.

Grenade Belt:
-Takes shield slot
-Allows player to throw bombs
-Decreases bomb radius

Why it's balanced:
Same as above. The lack of a shield balances out the ability to throw bombs and kite, and if that's not enough then a radius reduction also helps balance it out.