12-inch/50-caliber Mark 7 gun

The 12"/50 caliber Mark 7 gun (spoken "twelve-inch-fifty-caliber") was a United States Navy's naval gun that first entered service in 1912. Initially designed for use with the Wyoming-class battleship of dreadnought battleships, the Mark 7 also armed the Argentine Navy's Rivadavia-class battleships.

Design and development
The 12 in/50 caliber Mark 7 naval gun was only a slight improvement over the preceding American naval gun, the 12"/45 caliber Mark 5 gun. As such, it was a very similar weapon, having been lengthened by five calibers to allow for improved muzzle velocity, range, and penetrating power. Designed to the specifications of the Bureau of Ordnance, the Mark 7 was constructed at the U.S. Naval Gun Factory in Washington, D.C.

The Mark 7 weighed 124140 lb with the breech and was capable of firing two to three times a minute. At maximum elevation of 15° it could fire an 870 lb shell approximately 20000 yd. With an initial muzzle velocity of 2900 ft/s, the gun had a barrel life of 200 rounds, and was capable of firing either armor piercing or Common projectiles.

As designed, the Mark 7 was capable of penetrating 17.4 in of Harvey plated side armor at 6000 yd, 14.7 in at 9000 yd, and 12.3 in at 12000 yd. By comparison the 12"/45 caliber Mark 5 it replaced could penetrate 16.6 in, 12.2 in, and 9.9 in at those distances, respectively.

Bethlehem Steel built the first gun, No. 180. Mod 0, Nos. 181, 182, and 186–200, was a built-up gun consisting of a tube, jacket, and eight hoops, a screw-box liner with locking hoops and rings and hand operated and Smith-Asbury mechanism. The gun was constructed with nickel-steel and hooped to the muzzle.

Mod 1 was gun No. 180, rebuilt into gun No. 180L, with its chase hoops rebuilt along with a new conical nickel-steel liner, a smaller chamber, and the rifling increased.

Mod 2, gun Nos. 183–185, was a Mod 0 gun relined with a conical liner, a new chase locking hoop, and with a locking ring added. This brought the weight up to 125498 lb, with the breech. It also had a 25 cuin-smaller chamber.

The Mod 3 guns, Nos. 211–216, were the last new guns built, all other Mods were Mod 0, 2, or 3 guns that were modified. These guns were built with a new simplified design, no liner, five hoops, a locking ring, along with a screw-box liner and a different gas check seat.

Mod 4, the twelve guns from USS Wyoming (BB-32), relined in 1921–1923, had a conical one-step liner and uniform rifling with a new chase locking hoop and locking ring. With the Mod 5 an attempt was made to reline a Mod 1 with a uniform twist rifling, but it was dropped. Mod 6 relined Mod 2 with a uniform twist rifling along with a modified new chase hoop and locking ring. Mod 7 took the Mod 3 and used a one-step conical liner, uniform twist rifling, and added a tube and liner locking ring. Mod 8 was the Mod 0 or Mod 4 also using a one-step conical liner, uniform twist rifling that was secured by a tube and liner locking ring with a liner locking collar at the breech end. The Mod 8s that used Mod 0 guns also added a new chase hoop and locking ring. Mod 9 was a Mod 2 or Mod 6 that had a new liner with longitudinal clearances at the liner shoulders installed, uniform twist rifling along with a tube and liner locking ring and collar added at the breech end. Mod 10, like the Mod 9, was also a Mod 2 or Mod 6 that had a new liner with longitudinal clearance at the liner shoulders installed, uniform twist rifling along with a tube and liner locking ring and collar added at the breech end. The Mod 10 used Breech Mechanism Mark 9 instead of the Mark 8 on the previous Mods. Mod 11 was a Mod 7 that had the chamber lengthened, adding 235 cuin, and a 3½° breech band seating slope and used Breech Mechanism Mark 12. Mod 12 used a Mod 10 and lengthened the chamber and added a 3½° breech band seating slope with Mod 13 being similar but of a Mod 8, Mod 14 used a Mod 9, Mod 15 used a Mod 7, Mod 16 used a Mod 10, Mod 17 used a Mod 8, and Mod 18 a Mod 9. Mod 19, the last modification, used a Mod 2 with its breech modified for the Smith-Asbury Breech Mechanism and the forward end of the chamber modified similar to the Mod 18, lengthened the chamber and added a 3½° breech band seating slope. The breech end was further modified by being machined out so that it could accommodate a gas check seat liner locking ring. The Mod 19 could also be used right or left handed by cutting a new slide keyway that was 180° from the original keyway.