This designation was given to five guns manufactured by Bethlehem in 1906 that fired semi-fixed ammunition and were intended for capital ship secondaries. These guns did not enter regular naval service but instead were immediately placed into reserve where they remained until early 1917 when they were used to arm S.S. St. Louis for a brief time and were then returned to storage. Overall length was 270 in (6.858 m), bore length was 263.3 in (6.688 m) or 43.88 calibers. Gun weight was 15,340 lbs. (6,958 kg) including the breech. Mod 0 had 24 rifling grooves while Mod 1 had 36 grooves. Fired a 105 lbs. (47.6 kg) shell. With a 20 lbs. (9.1 kg) charge, muzzle velocity was 2,250 fps (686 mps), range was 19,000 yards (17,370 m) at 30.3 degrees elevation and pressure was 15 tons/in² (2,360 kg/cm²).

During practice firing on S.S. St. Louis in 1917 these guns experienced several in-bore and out-of-bore prematures. The prematures were eventually traced to the brass cups used to seal the mouth of the propellant cartridges. Upon firing these cups could strike and damage the base fuze of the projectile. Wooden plugs were quickly substituted and these solved the problem.

A gun built by Vickers Sons & Maxim Co., no details survive. May have been purchased to evaluate British construction methods and gun performance. Vickers serial number 1015a, USN serial number 523.

A wire-wound gun built by Vickers Sons & Maxim Co., no details survive. May have been purchased to evaluate British construction methods and gun performance. Vickers serial number 1070a, USN serial number 524.

Depth charge gun. Redesignated as Depth Charge Projector Mark 2 Mod 0.