6-inch/50-caliber gun
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 and battleships, as well as the and armored cruisers. They were also used as the main battery on the protected cruisers. Design. The /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. 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 to . 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. 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. This gun is also unusual for the US Navy where the overall length of the gun barrel, , is used to measure the caliber of the gun. (i.e.:300.2/6=50) Normally the bore length, , would be used and this would actually be a 49 caliber gun. (i.e.:294/6=49) Naval Service. The "Maine"s 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. Coast defense locations. 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. Surviving examples. Surviving 6"/50 caliber naval guns include: 6-inch/50-caliber gun The