Photo of the B-52H's M61 Vulcan canon by Bob Nelson at the Grand Forks Air Force Base (GFAFB) 'Friends And Neighbors Day' on May 27th 1989.
The B-52H had its ECM and avionics updated, a new fire control system was fitted, and the rear defensive armament was changed from machine guns to a 20 mm M61 Vulcan cannon, later removed in 1991Ð94. (Wikipedia)