When you lack the minimum amount of ammo to fire a weapon, you should not be able to select the weapon in the first place. This includes weapons with two different sets of ammo (like the Strife crossbow).
Also, if a weapon has two different ammo and the primary ammo type is gone (for example, the crossbow w/electric arrows), it should automatically switch to that same weapon that uses an available ammo type. (Using the Strife crossbow example, you switch to the poison arrows if you don't have any electric arrows left.)
Edit: Only applicable to Strife
Edit2: Update version number.