The second of three mobile suit prototypes built under the Earth Federation's "Project V" during the One Year War, the RX-77 Guncannon, the Federation's first fully humanoid mobile suit design, is a mid-range support unit with shoulder-mounted cannons and heavy Luna Titanium armor. Like its brethren Guntank and RX-78-2 Gundam, the Guncannon used the same FF-X7 Core Fighter as an escape system for the pilot. Also like the Gundam, the Guncannon was one of the first mobile suits to use the energy cap-based beam rifle, as well as head-mounted vulcan guns for close range defense.

After one prototype unit had been completed, the design was revised to permit the use of beam weapons. The result was the pre-production RX-77-2 version, armed with a powerful beam rifle; since the mobile suit wasn't intended for close combat, it carried no melee weapons. However, the Guncannon was designed as a mid-range fire support unit, and its primary armament was its shoulder-mounted 240 mm cannons. The main drawback to this was the cannons' limited ammunition supply, but an experienced pilot learned to use the ammo conservatively and effectively to stay in battle longer.

Source: Gundamproject.com, HG-UC kit manual