So anyone have any ideas how to give a custom gun a ammo limit? That can be refilled with more ammo? Or is this impossible?
Well, Barrett's Armory Mod, I think, is close to what you are looking for. When using those guns, I believe you have to "reload" after so many shots (dependent on the gun). The reload is done by right-clicking (using the weapon's alternate ability). That would be a start, anyway. Next step would be making ammo a prerequisite (and have it be consumed) for using the reload. This part would be trickier, I imagine.