The 3"/21 caliber gun (spoken "three-inch-twenty-one-caliber") was a field gun for United States Navy and United States Marine Corps. They were a simple horse-drawn artillery gun that were mostly used by the Marines. The guns have also been described as Boat Guns but information on type of mounts has not been found.^[1]

The Mark 1 was intended as a support gun for landing operations. It was of simple monobloc construction with a Fletcher breech mechanism with down swinging carrier, and used a hydraulic recoil piston that was screwed onto the gun directly. It most likely used bagged ammunition with a 13 pounds (5.9 kg) projectile at a velocity of 1,150 feet per second (350 m/s). Range was 5,000 yards (4,600 m) at 19.5 degree s of elevation.^[1]

Guns No. 5 and 6, built in 1875, had been restored and were located in St. Clair, Pennsylvania as of 2004.^[1]