5-inch/51-caliber gun

The different marks of the gun were Marks 7, 8, 9, 14, and 15. 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 lb (11.1 kg) charge of smokeless powder gave a 50-pound (23 kg) projectile a velocity of 3,150 ft/s (960 m/s). Range was 15,850 yards (9.0 statute miles or 14.5 kilometres) at the maximum elevation of 20 degrees.[2] Useful life expectancy was 900 effective full charges (EFC) per liner.[3]

On a US Navy transport ship c. mid 1942

The 5-inch/51 caliber gun was designed to engage destroyers, torpedo boats, and other surface targets. The 5"/51 gun entered service in 1911 as secondary armament on the Florida-class battleships, which mounted 16. The guns served well through World War I, but 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 some of the World War I-era battleships were rearmed with dual purpose guns as well. Surplus 5"/51 guns from scrapped or rearmed battleships were mounted in United States Coast Guard cutters, auxiliaries, small aircraft carriers, coast defense batteries, and Defensively Equipped Merchant Ships.[3] A 1939 Table of Organization and Equipment shows Marine defense battalions were equipped with six of these guns each.[4] 5-inch/51 shore batteries were used with great effectiveness by the 1st Marine Defense Battalion during the Battle of Wake Island in December 1941. These were replaced in the defense battalions by the 155 mm Long Tom gun by 1943. Six Tambor-class submarines were rearmed with "wet mount" 5-inch/51 guns during World War II, taken from Barracuda-class submarines or spares for that class.

The 5"/51 caliber gun was mounted on:

5"/51 caliber ex-Navy guns were emplaced during World War II at several locations, some operated by the United States Army Coast Artillery Corps and some by Marine defense battalions. This list may not be exhaustive. They were grouped into two-gun batteries unless otherwise noted.[25]

• Two guns near Cape Lookout, NC
• Three guns in Battery Gillespie, Point Loma, San Diego, CA
• Three guns in Battery Ahua, Fort Kamehameha, Oahu, HI
• Two guns in Battery Nanakuli, Oahu, HI
• Two guns in Battery Oneula (Ewa), Oahu, HI
• Two guns at Kahana Bay, North Shore, Oahu, HI

In British service these guns were known as 5"/51 BL Mark VI and Mark VII. During World War I three of these guns formed part of the coastal defences of Scapa Flow.[2] 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 (at least six, possibly more) and deployed ashore for coastal defence.[2]

Surviving 5"/51 caliber guns include:[26]

• Eight guns on USS Olympia (C-6), preserved at the Independence Seaport Museum in Philadelphia, Pennsylvania (guns previously on USS Colorado (BB-45))
• Six guns preserved on USS Texas (BB-35) near Houston, Texas
• One Mark 8 gun (Four Lakes #1205) at Trumbo Point, Key West, Florida (part of Naval Air Station Key West)
• One gun (Unk. mfr. #1093L2) at the Ropkey Armor Museum, Crawfordsville, Indiana (previously on USS Arizona (BB-39) and allegedly on USS Indiana (BB-1) or USS Indiana (BB-58) at some time)
• Two Mark 7 guns (Watervliet #774 and #Unk.) on Midway Island, Central Pacific Ocean
• One Mark 7 gun (Naval Gun Factory (NGF) #415L) at the NROTC facility, Tulane University, New Orleans, Louisiana
• One Mark 15 gun (NGF #736L) at the U.S. Navy Museum, Washington Navy Yard, Washington, DC (previously on USS Idaho (BB-42))
• One 5"/51 caliber gun at Fort Schuyler, Bronx, New York (possibly at USMMA, Kings Point, New York)
• One 5"/51 caliber gun at Treasure Island, San Francisco, California (behind museum)
• One Mark 15 gun in Lewiston, Maine
• One Mark 15 gun (Bethlehem #Unk.) at the Brunswick Executive Airport, Brunswick, Maine (formerly NAS Brunswick)
• One 5"/51 caliber gun in Mitchell, Indiana
• One Mark 9 Mod 3 gun (NGF #938L), American Military Museum, South El Monte, California
• One 5"/51 caliber gun at the Veterans Memorial Museum, Chehalis, WA

• Bauer, K. Jack; Roberts, Stephen S. (1991). Register of Ships of the U.S. Navy, 1775-1990: Major Combatants. Westport, Connecticut: Greenwood Press. ISBN 0-313-26202-0.
• Berhow, Mark A., ed. (2015). American Seacoast Defenses, A Reference Guide, Third Edition. McLean, Virginia: CDSG Press. ISBN 978-0-9748167-3-9.
• Breyer, Siegfried (1973). Battleships and Battlecruisers 1905–1970. Doubleday and Company. ISBN 0-385-07247-3.
• Campbell, John (1985). Naval Weapons of World War Two. Naval Institute Press. ISBN 0-87021-459-4.
• Fahey, James C. (1939). The Ships and Aircraft of the U.S. Fleet. Ships and Aircraft.
• Fahey, James C. (1941). The Ships and Aircraft of the U.S. Fleet, Two-Ocean Fleet Edition. Ships and Aircraft.
• Fairfield, A.P. (1921). Naval Ordnance. The Lord Baltimore Press.
• Friedman, Norman (1983). U.S. Aircraft Carriers. Naval Institute Press. ISBN 0-87021-739-9.
• Preston, Antony (1980). Cruisers. Prentice Hall. p. 60. ISBN 0-13-194902-0.
• Friedman, Norman (2011). Naval Weapons of World War One. Seaforth Publishing. ISBN 978-1-84832-100-7.

• List of all US coastal forts and batteries at the Coast Defense Study Group, Inc. website
• FortWiki, lists most CONUS and Canadian forts