8"/45 caliber gun

The 8"/45 caliber Mark 6 gun (spoken "eight-inch-forty-five--caliber") were used for the secondary batteries of the United States Navy's last pre-dreadnought battleships and refitted in older armored cruisers primary batteries.

Design
The 8-inch/45 caliber gun was developed after the Spanish-American War to use the new smokeless powder that had recently been adopted by the Navy. This gun was much stronger than its predecessor, the 8"/40 caliber gun, which were incapable of handling the new powder. This was shown when the muzzle of one of USS Colorado (ACR-7)'s guns blew off on 22 June 1907, during gunnery practice off Shantung. The Mark 6, gun Nos. 108 – 255, 148 in total, was constructed of tube, jacket, four hoops a locking ring and the liner with a Welin breech block. These were all constructed of nickel steel. There were a tolal of eight different Mods, Mark 6 Mod 0 to Mark 6 Mod 7, with different liners, breech mechanisms, chambers, and rifling being used.

Service history
The guns mounted in the Virginia-class battleships were in an unusual two-level turret with the 8-inch guns on top of the larger 12-inch guns. This arrangement ultimately proved unsuccessful but helped the Navy in in the successful development of superfiring turrets later used in the dreadnought USS South Carolina (BB-26).

Due to an older 8"/40 caliber Mark 5's muzzle blowing off during gunnery practice in USS Colorado (ACR-7) on 22 June 1907, all Mark 5's were removed from service, rebuilt, and placed in reserve. Because of this, all armored cruisers and the armored cruiser USS New York (ACR-2), were refit with the newer Mark 6 guns.

With the signing of the Washington Naval Treaty, the pre-dreadnoughts still in service were required to be scrapped. This surplussed about two dozen guns, which the Army used for coastal artillery, using new mountings and new lighter, and more streamlined, projectiles.