In addition to deployable equipment, Halo 3 seems to have a new category of "heavy weapons" that have special characteristics.

One is a variation on the fixed turret-mounted machine guns we're familiar with from Halo 2. They can be used from a fixed position. However, pressing the B key removes the gun from its turret so you can carry it with you. In this configuration it has limited ammunition, and also enforces other limitations on you.

While wielding a heavy weapon, you cannot zoom, melee, throw grenades, dual wield, drive or ride in vehicles, or crouch. Your foot speed is limited to a slow walk, and your turning speed is likewise reduced. Using the gun puts you in a third person perspective with the camera over your shoulder, similar to driving a vehicle.

Wielding a weapon like this perhaps makes Halo 3 feel a bit like Gears: your movements are slow and ponderous, and you get to look at all the cool armor and weapons up close.

NOTE: Information from the beta version subject to change!