Bungie has confirmed a new weapon called a Railgun.
The official designation for the power weapon is the Asymmetric Recoilless Carbine-920 (aka ARC-920) Railgun and it fires an explosive round at ultra-high speeds to deliver a near-instantaneous kinectic and explosive blow to both vehicles and infantry. It's probably best to think of it as a smaller version of the Warthog Gaussgun.
While the Railgun can take out a soft target in one or two shots, it is not as powerful against vehicles as the Spartan Laser. Instead, it is more like the Brute Shot in that its high-speed round can alter a vehicle's direction and possibly flip it over or send it off a cliff.
Currently, the Railgun has to be charged before firing but it only takes approximately half as long as the Spartan Laser. Also, while it does not have a zoom function your aim isn't altered while getting shot at like other zoomed weapons in the Halo series.