Multifunctional Ammo

Overview

Often used in old-school arcade shooters such as Unreal Tournament, multi-functioning ammo can be used for many purposes. Some of these purposes may just be for ease of use while playing a game (so you don't have to constantly switch weapons).  It can also be used for other reasons, such as using ten bullets to make a grenade like in the first soldier of fortune.
 
In the left corner an example of multi-functioning ammo, the rocket launcher can use its own rockets to fire grenades, notice, the rocket have wings deployed and the engine ignited and the grenade have no wing deployed and the engine is off (the rocket will explode on impact, and the grenade will bounce and explode after a few seconds) In Quake there is the same thing however, rockets deserves to act like rockets in the rocket launcher and grenades in the grenade launcher. The concept was pushed further in the game gunman chronicles where rockets could work like grenades, laser triggered mines, missiles, and even having different payloads like cluster and explosive.

This concept doesn't include energy weapons which are often naturally flexible by nature, and weapons that can use anything in the world like the HL2's gravity gun