Halo 3/Weapons

Spartans are trained to use any weapon found on the battlefield — Human or otherwise. This training and philosophy means that resupply is always just one victory away. Efficient and deadly, Spartans often look at opposing forces as a kind of mobile weapons cache.

Grenades
You can carry a maximum of two of each grenade type at a time. Press to switch between available grenade types when not dual-wielding. Note that in the Campaign, grenades explode upon melee (or any other form of damage).

Support weapons
Halo 3 introduces the idea of Support weapons, large weapons that require the player to be controlled in third-person view. Players cannot melee, nor throw grenades while using these weapons. A turret is typically activated by pressing. A Spartan, however, is strong enough to tear off these weapons (press  while using the turret) and use them almost like rifles. When detached, these weapons have limited ammunition capacity.

Other weapons
These objects can be used in melee attacks. Note that these weapon will kill anyone instantly and are used mostly in multi-player modes, aside from the hidden skulls in campaign mode. To pick these up, hold. To attack, press.