I think you it shouldn't have a cap. If it has no cap then the player would have to pick it up on their own and it wont be unlimited because it is unlimited. Their limit depends on how much the player picks up. This should also not be tradable then since players can easily rack up ammo from trades.

Whereas if a player doesn't want to take the time to pick up ammo then they may purchase VIP or buy the unlimited ammo for a limited time.