5"/51 caliber gun

5"/51 caliber guns (spoken "five-inch-fifty-one-caliber") formed the main battery of the first United States Navy light cruisers and the secondary batteries of United States Navy battleships built from 1907 through the 1920s. United States naval gun terminology indicates the gun fired a projectile 5 inches (127 mm) in diameter, and the barrel was 51 calibers long (barrel length is 5" × 51 = 255" or 6.4 meters).

Description
The built-up gun consisted of a tube, full-length jacket, and single hoop with side swing Welin breech block and Smith-Asbury mechanism for a total weight of about 5 metric tons. Some Marks included a tapered liner. A 24.5-pound (11 kg) charge of smokeless powder gave a 50-pound (23 kg) projectile a velocity of 3,150 feet per second (960 m/s). Range was 15,850 yards (9 statute miles) (15 km) at the maximum elevation of 20 degrees. Useful life expectancy was 900 effective full charges (EFC) per liner.

US service
Increased awareness of the need for anti-aircraft protection (especially following the attack on Pearl Harbor) encouraged mounting of dual-purpose 5"/38 caliber guns in later battleships and most of the World War 1-era battleships were rearmed with 5"/38 caliber guns or 5"/25 caliber guns during World War 2. Surplus guns from scrapped or re-armed battleships were mounted in United States Coast Guard cutters, auxiliaries, small aircraft carriers, coast defense batteries, fleet submarines, and Defensively Equipped Merchant Ships. 5"/51 shore batteries were used with great effectiveness by the Marines during the Battle of Wake Island in December 1941.

The 5"/51 caliber gun was mounted on:
 * USS Olympia (C-6) - replacing the original 10 5"/40 and 4 8"/35 guns
 * USS Chester (CL-1)
 * USS Birmingham (CL-2)
 * USS Salem (CL-3)
 * USS Florida (BB-30)
 * USS Utah (BB-31)
 * USS Wyoming (BB-32)
 * USS Arkansas (BB-33)
 * USS New York (BB-34)
 * USS Texas (BB-35)
 * USS Nevada (BB-36)
 * USS Oklahoma (BB-37)
 * USS Pennsylvania (BB-38)
 * USS Arizona (BB-39)
 * USS New Mexico (BB-40)
 * USS Mississippi (BB-41)
 * USS Idaho (BB-42)
 * USS Tennessee (BB-43)
 * USS California (BB-44)
 * USS Colorado (BB-45)
 * USS Maryland (BB-46)
 * USS West Virginia (BB-48)
 * USS Barracuda (SS-163)
 * USS Bass (SS-164)
 * USS Bonita (SS-165)
 * USS Langley (CV-1)
 * USS Long Island (CVE-1)
 * USS Sangamon (CVE-26)
 * USS Suwannee (CVE-27)
 * USS Chenango (CVE-28)
 * USS Santee (CVE-29)
 * USS Charger (CVE-30)
 * Bogue class escort carriers
 * Tambor class submarines
 * USCG Tampa Class Cutters
 * USCG Lake Class Cutters
 * USCG Treasury Class Cutters
 * Banff class sloops

British service
During World War I three of these guns formed part of the coastal defences of Scapa Flow. In World War II a small number of these guns entered British service on board ships transferred under the Lend-lease arrangement. Some of these guns were then transferred to New Zealand and deployed ashore for coast defense.