The Convair B-58 Hustler's tail cone that enclosed the tail gun was composed of tapered, concentric aluminum rings nested within each other and spring-loaded against each other to form a flexible aerodynamic shell for the gun system.
After looking at various options including rear-firing missiles and twin 30mm cannons, the General Electric T171 (first designation of the M61 Vulcan cannon) 20mm rotary cannon was selected in early 1954. The tail gun of the B-58 boasted many firsts for an aircraft defensive gun including the first fully automatic defensive gun fire control system for a production aircraft, the first aircraft gun unit to use a 3-axis inertially-stabilized platform to increase the gun's accuracy, the first aircraft gun to use a self-contained environmental control unit for the gun and ammunition storage, first aircraft gun to use a solid state analog fire control computer for fire control and the first to use a hinged turret arrangement for maintenance work.
The forward muzzle velocity of the 20mm T171 cannon was lower than the forward velocity of the B-58 when it was flying at Mach 2. So relative to the ground, the cannon round would be moving backwards when leaving the muzzle!
Source: Convair B-58 Hustler: The World's First Supersonic Bomber by Jay Miller. Midland Publishing/Aerofax, 1997, p104-105.