Ammo warning

Known variants:
* Text or icon appears on HUD
* A light or color change in in-game weapon or HUD appears (e.g. ammo counter turns red or a red light lights near the magazine or wherever the user can see while shooting)
* A sound is emitted with last few shots (commonly a click-click which is similar to when trying to fire an empty gun).
* A voice-over warns of it (implies AI or smart weapons system and integration with you or your other equipment)
* Physical changes with the magazine or equivalent, e.g. end of belt, magazine moves when shooting, etc. so you see when it's low by its position.