According to the NavWeaps.com website, by 1944/1945, the US Navy found that their standard 20mm and 40mm AA guns were not effective in stopping kamikaze attacks, which led to an accelerated program to develop an intermediate-caliber weapon (bigger than the 40mm, but smaller than the 5in/38 cal) that could fire a VT fused shell.  That led to development the 3"/50cal Mk 22 as an automatic weapon.  Even though this weapon was rushed through the design phase, the prototype wasn't test fired until September 1, 1945.  The close of the war slowed development, but led to an improved 3"/70cal design.  Fleet wide delivery of this new weapon didn't begin until 1948, with some weapons remaining in service on US ships until as late as the 1980's. 

These were big mounts - the mounting was the same size as a 40mm quad Bofors.  The new weapons were swapped out for the 40mm quads on a one-to-one basis.  According to some sources, one of these weapons, mounted on the USS Biddle (DLG-34) damaged a Mig fighter in the Tonkin Gulf in 1972.

For more on this weapons, click the link above. 

As mentioned, there are four mounts included included.  The detail and fineness of these weapons is superb.  The barrels, side rails and sight especially stand out to me. 

This is Black Cat Models set AC700010, Mk.33 3" Automatic Twin Gun (x4).  It retails for about $6.25 USD, and is available directly from Black Cat Models - who I'd like to thank for this review sample - or from some of our other fine sponsors.