4"/50 caliber gun

The 4"/50 caliber Mark 9 gun (spoken "four-inch-fifty-caliber") was the standard low-angle, quick-firing gun for United States destroyers through World War I and the 1920s. United States naval gun terminology indicates the gun fired a projectile 4 in in diameter, and the barrel was 50 calibers long (barrel length is 4 inch x 50 = 200 inches or 5 meters).

Description
The built-up gun with a tube, full-length jacket, and side swing Welin breech block with Smith-Asbury mechanism weighed about 2.7 tons. Fixed ammunition (case and projectile handled as a single assembled unit) with a 14.5 lb charge of smokeless powder gave a 33 lb projectile a velocity of 2900 ft/s. Range was 9 mi at the maximum elevation of 20 degrees. Useful life expectancy was 500 effective full charges (EFC) per barrel.

Increasing awareness of the need for improved anti-aircraft protection encouraged mounting of dual purpose guns on destroyers beginning in the 1930s. The dual-purpose 5"/38 caliber gun became standard for United States destroyers constructed from the 1930s through World War II. United States destroyers built with 4"/50 caliber low-angle guns were rearmed with dual-purpose 3"/50 caliber guns.  The 4"/50 caliber guns removed from destroyers were mounted on British Defensively Equipped Merchant Ships and American armed merchant cruisers ('auxiliary cruisers') like SS Stephen Hopkins.

US Navy service
The 4"/50 caliber gun was mounted on:
 * Caldwell class destroyers
 * Wickes class destroyers
 * Clemson class destroyers
 * Town class destroyers
 * United States S class submarines
 * the first seven Balao class submarines
 * USS Dolphin (SS-169)
 * rearmed submarines USS Salmon (SS-182), USS Seadragon (SS-194), USS Gato (SS-212), USS Silversides (SS-236) and USS Robalo (SS-273)

UK service
Many Mark 9 guns were supplied to the United Kingdom during World War II as part of Lend-lease, both individually and on naval and merchant ships.