McDonnell F-4D 'Phantom II' flown by The Happy Hooligans from the NDANG at Hector Field, Fargo, ND.
Photo by Bob Nelson

Belly mounted pod: Vulcan Gattling 20mm Machine Gun.
SUU-23/A
0428
Remote Cooling For Ground Firing. For Field Maintenance Instructions See T.O. 11z1-31-5-2

This pod featured a self-powered variant of the M61A1 by the US Army and GAU-4/A by the US Air Force. This modification allowed its carriage on aircraft that could not meet the speed requirement of the previous unit, and reduced drag by removing the ram-air turbine requirement. This pod was popular for use on the F-4C and F-4D Phantom II aircraft. The pod still has a weight restriction, weighing more than its predecessor at 1,730 lbs loaded with 1,200 rounds of ammunition, and still has the fixed rate of 6,000 rpm. -Wiki