One outstanding candidate for this was the Swiss Oerlikon company's 20mm gun, which was in service with Great Britian and which had been reviewed for service with the USN before, but rejected on grounds of low rate of fire. Eventual adoption of the 20mm may have been due to the British who had lost their contact with Switzerland in 1940 and wanted a U.S. production line. Eventually, on the grounds of it being the best available weapon, BuOrd approved the production in November 1940.

Reception was enthusiastic as a more than welcome replacement for the .50 caliber, production was slow to quickly replace all of the .50 Cal, so in the early months most air-defenses continued to rely on the .50-caliber.

The 20 millimeter guns were the smallest anti-aircraft guns carried aboard the USS Slater. Heavy machine guns, they were capable of firing 450 rounds per minute per barrel with an approximate range of 2000 yards. Each gun was manned by a crew of 4; a gunner, sight(range) setter, one loader per barrel. The gun was originally designed in Switzerland by the Oerlikon Company, hence the metric designation. Original mounts were on trunions, and required another crew member to raise and lower the trunion. Later installations were the Mk 10 (tripod) mounts that eliminated this crew man.