Army tank and anti-tank guns. Fourteen guns were used by the Navy of which five were mounted in Daimler armored car turrets on LCS (L) 201-205. These guns had removable breech rings and semi-automatic vertically sliding breech mechanism. Mark X differed from Mark IX only in that it used an autofretted barrel. Weight with breech mechanism was 287 lbs. (130 kg). Muzzle velocity with a 2.72 lbs. (1.23 kg) AP shot was 2,650 fps (808 mps).
Converted sub-caliber guns with a single tube barrel, breech ring and horizontal radial shaped breech block. Weight with breech mechanism was about 132 lbs. (60 kg). Muzzle velocity with a 2.0 lbs. (0.91 kg) projectile was about 1,200 fps (366 mps). Mounting was Mark IX which was a pedestal type that used rubber buffers to absorb the recoil and had a maximum elevation of +70 degrees. A total of 180 guns were converted and these were used as emergency scatter guns on HDMLs (Harbor Defence Motor Launch) and small auxiliaries. Case shot was provided as well as HE projectiles.
Converted sub-caliber guns with a single tube barrel, screwed and shrunk breech ring and horizontal radial shaped breech block. Weight with breech mechanism was about 170 lbs. (77 kg). Muzzle velocity with a 2.0 lbs. (0.91 kg) projectile was about 1,920 fps (585 mps), similar to the Mark VIII firing low-velocity projectiles. Mounting was Mark IX, described above. 115 guns were converted and these were used on HDMLs and small auxiliaries. Actual bore length was 39.37 calibers, same as the Mark II and Mark VIII.
Semi-automatic guns designed by Rolls-Royce and used mainly on MLs (Motor Launch). These guns were considered unsatisfactory, at least initially, although 602 were built. Used a monobloc barrel, breech ring and breech body in which the semi-automatic breech block moved horizontally. Casings and cradle were aluminum alloy. Ballistics were similar to the Mark VIII. Mounting was Mark XIV which was a pedestal type with a elevation range of -12 to +60 degrees. Recoil was 17.5 inches (44.5 cm). Gun and cradle weighed 336 lbs. (152 kg), these together with the mounting totaled 1,036 lbs. (470 kg).