a different groove pattern. The Mark 3 was the same as the Mark 2 but used a one-step conical liner. The Mark 3 Mod 0 had an increasing rifling twist (like the Mark 2 Mod 0) while the Mark 3 Mod 1 utilized a uniform twist. At the time the program was cancelled, in 1922, 71 guns had been built, including the prototype, while another 44 were in progress. A Mark 3 Mod 1 was modified and used as the prototype for the 16-inch/50 caliber Mark 7 gun, which would go on to arm the Iowa-class battleships; it was redesignated as Mark D Mod 0. Description These built-up guns were long—50 times their bore, or 50 calibers from breechface to muzzle. With a full powder charge of , the guns were capable of firing a Mark 3 armor-piercing shell with a muzzle velocity of firing out to an effective range of . The Army used a reduced charge ( with Mark 3 shell or with Mark 12 shell) and either a Mark 3 shell or a Mark 12 shell, for a muzzle velocity of with the Mark 3 or with the Mark 12. The range for the Army version is listed as but it is not clear which shell and charge this is for. Service With war on the horizon, the Navy released the approximately 50 remaining guns, and on 27 July 1940, in the wake of the Fall of France, the Army's Harbor Defense Board recommended the construction of twenty-seven 16-inch two-gun batteries to protect strategic points along the US coastline, to be casemated against air attack, as was begun with almost all of the older batteries. The M2 through M5 barbette mounts were used for the later batteries. As with the previous M1919 barbette carriage, these were designed with an elevation of 65 degrees to allow plunging fire as enemy ships approached. About twenty-one 16-inch gun batteries were completed at eleven harbor defense commands in 1941–44, but not all of these were armed. Typical of this plan were the guns placed to protect Narragansett Bay, Rhode Island; two 16-inch guns in Battery Gray, Fort Church, Little Compton, Rhode Island, with two more in Battery Hamilton, Fort Greene, Point Judith, Narragansett, Rhode Island. A second battery of 16-inch guns at Fort Greene, Battery 109, had construction suspended in 1943 and never received guns. These batteries were placed such that