The Whirlwind essentially consists of two 20mm rotating barrel gun systems, which when fired together can inflict damage per a +8 weapon with each and every deadly burst (use Agility for the purposes of trumping and such). The Whirlwind is towable by any vehicle with a hitch on it (it cannot drive by itself), that has room for only one person - in the firing chair. It's equipped with these convenient little extras:

Carbon Graphite Armor Plating: since the Whirlwind operator is often using the thing while under fire (and heavy fire, at that), the Twin Battle Gun is equipped with a massive, inch thick plate of carbon graphite armor, armor which provides a body using this weapon +6 protection from frontal attacks - of course, this armor is ineffective against flank and rear assault.

Collapsible Tow Wheels / Isolation Pads: the wheels of the Whirlwind can fold down while its not being towed, allowing them to prevent the Twin Battle Gun from rolling back from its firing recoil. This is also enhanced by a main firing brace at the front of the gun, which essentially mounts the gun in a sort of tripod configuration while it's used in a stationary fashion.

Computer Enhanced Target Display Panel: the Whirlwind's onboard threat computer helps a Joe using the Twin Battle Gun to eliminate the enemy more efficiently. The thing effectively adds to the gunner's chance to hit an opponent by +1, which helps in a pinch (especially with the Joes' tendency to use the Whirlwind while it's being towed around...).

Hydraulic Bio-Form Command Seat: this advanced chair allows the Whirlwind operator to easily adjust his or her body for maximum comfort while the Twin Battle Gun swivels about, since it can do so in a full 360 degrees, and with about a 45 degree arc of fire. The command seat doesn't do anything else, really, but it does sound pretty darn cool...