6"/50 caliber Mark 6 and Mark 8
USS Ohio (BB-12), 6-inch/50 caliber guns.
Type	Naval gun Coastal defence
Place of origin	United States
Service history
In service	1903
Used by	United States Navy
Wars	World War I World War II
Production history
Designer	Bureau of Ordnance
Designed	1898
Manufacturer	U.S. Naval Gun Factory Midvale Steel
Produced	1900–1917
No. built	Mark 6: 136 (Nos. 197, 210–259, 277–359, 421–422) Mark 8: 215 (Nos. 360–420, 427–510, 525–594)
Variants	Mark 6 and Mark 8
Specifications
Mass	18,112 lb (8,215 kg) (without breech) 18,628 lb (8,450 kg) (with breech)
Length	300.2 in (7,630 mm)
Barrel length	294 in (7,500 mm) bore (49 calibers)
Shell	105 lb (48 kg) naval armor-piercing
Caliber	6 in (152 mm)
Elevation	−10° to +15°
Traverse	−100° to +100°
Rate of fire	6 rounds per minute
Muzzle velocity	2,800 ft/s (850 m/s)
Effective firing range	15,000 yd (14,000 m) at 14.9° elevation WWI charge 16,000 yd (15,000 m) at 15° elevation WWII charge

The 6"/50 caliber gun Mark 6 and Mark 8 (spoken "six-inch-fifty-caliber") were used for the secondary batteries of the United States Navy's Maine-class and Virginia-class battleships, as well as the Pennsylvania-class and Tennessee-class armored cruisers. They were also used as the main battery on the St. Louis-class protected cruisers.^[1][2]

The 6-inch (152 mm)/50 caliber Mark 6 guns were developed around the time of the Spanish–American War. The gun card designates the gun as 46-caliber but the Bureau of Ordnance lists the guns as 50-caliber.^[1][2]

The Mark 6 gun was constructed of gun steel using a tube, jacket, four hoops, a locking ring, and Welin breech block. It was discovered that the pressure curve of the charge and the strength curve of the barrel match exactly along one point of the barrel, because of this the muzzle velocity was reduced from the original 2,800 ft/s (850 m/s) to 2,600 ft/s (790 m/s). With the Mod 1 nickel-steel was used for the tube, two hoops and the locking ring, the gun was also hooped to the muzzle, because of this, the original muzzle velocity was able to be restored. The Mod 2 were Mod 0s with a hoop added to the chase along with a cylindrical liner made from nickel-steel. With these changes the original muzzle velocity was used.^[1][2]

The Mark 8 was similar to the Mark 6 Mod 1 but was constructed entirely out of nickel steel. The Mod 1 had a slightly smaller diameter chamber, but the same volume. The Mod 2 had its rifling grooves increased to 36 from 24 and didn't have the modified chamber of the Mod 1. The Mod 3 had a cylindrical liner made of nickel-steel along with a new chamber design and 36 grooves. Mod 4 used an earlier gun that had its liner replaced with a conical nickel-steel liner along with 36 grooves and the chamber of the Mod 3.^[1][2]

This gun is also unusual for the US Navy where the overall length of the gun barrel, 300.2 in (7,630 mm), is used to measure the caliber of the gun. (i.e.:300.2/6=50) Normally the bore length, 294 in (7,500 mm), would be used and this would actually be a 49 caliber gun. (i.e.:294/6=49)^[1]

The Maines had their guns reduced to eight in 1909, the guns would go on to arm auxiliary ships during World War I and again in World War II. After WW I, and as a result of the signing of the Washington Naval Treaty, many of these ships were scrapped; the guns were later put to use as coastal artillery.^[1]

6"/50 caliber ex-Navy guns were emplaced during World War II at numerous locations; most were operated by the United States Army Coast Artillery Corps. This list may not be exhaustive. They were grouped into two-gun batteries unless otherwise noted. Most of the batteries in CONUS were disestablished in 1943 as new defenses were built or the threat abated.^[3]

• Two guns near Fort Macon, Beaufort, NC
• Two guns near Freeport, TX
• Two guns in Battery Lobos, Fort Miley, San Francisco, CA^[4]
• Six guns in a two-gun and a four-gun battery, temporary defenses of Grays Harbor, Westport, WA
• Two guns in Battery Allen, Fort Babcock, Sitka, AK
• Two guns each in Alaska at Cold Bay, Chernofski, Umnak, George Island (one gun), Yakutat, Nome, Annette Island, Adak Island, Shemya Island, and Popof Island
• Two guns each in Batteries 954 and 604, Shalloway Point and Latine Point, Fort McAndrew, Argentia, Newfoundland
• Four guns in two batteries at Blunt's Point and Breakers Point, Pago Pago, American Samoa
• Four guns at Paramaribo, Suriname (formerly Dutch Guiana)

Surviving 6"/50 caliber naval guns include:^[3]

• One Mark 8 gun, Naval Gun Factory 368, Nome airport, AK
• Two guns, Cannon Beach, Yakutat, AK
• One gun, city dump, Cold Bay, AK
• One gun, George Island, Cross Sound, AK
• Two Mark 8 Mod 2 guns, Midvale Nos. 550 and 554, Blunts Point, Pago Pago, American Samoa
• Two Mark 8 Mod 2 guns, Breakers Point, Pago Pago, American Samoa
• Four guns, Paramaribo, Suriname (formerly Dutch Guiana). One is a Mark 6 Mod 1, No. 309L, another is a Mark 6 Mod 2, No. 314L
• One gun presumably from USS Georgia (BB-15) alongside the ship's bell, Georgia Institute of Technology, Atlanta, GA.^[5]

• 6"/30 caliber gun - includes 6"/40 caliber Mark 3 guns
• 6-inch gun M1897 - Army coast defense weapon of similar type and era

• BL 6 inch Mk XI naval gun British equivalent
• 15 cm/50 41st Year Type Japanese equivalent

• Berhow, Mark A., ed. (2015). American Seacoast Defenses, A Reference Guide (Third ed.). McLean, Virginia: CDSG Press. ISBN 978-0-9748167-3-9.
• Friedman, Norman (2011). Naval Weapons of World War One. Seaforth Publishing. p. 180. ISBN 978-1-84832-100-7.
• "United States of America 6"/50 (15.2 cm) Mark 6 and Mark 8". Navweaps. 2 May 2015. Retrieved 22 September 2016.

• Bluejackets Manual, 1917, 4th revision: US Navy 14-inch Mark 1 gun
• List of all US coastal forts and batteries at the Coast Defense Study Group, Inc. website
• FortWiki, lists most CONUS and Canadian forts