12-inch coast defense mortar

M1890MI
An M1890 M1 12-inch mortar elevated to firing position. The mortar in the background has been depressed to loading position.
Type	Coastal artillery Railway gun
Place of origin	United States
Service history
In service	1895–1945
Used by	United States Army
Wars	World War I World War II
Production history
Designer	Bureau of Ordnance
Designed	1890
Manufacturer	Watervliet Arsenal
Specifications
Weight	29,120 lb (13,210 kg) (with breech)
Length	161 in (4,100 mm)
Barrel length	141.125 in (3,584.6 mm) bore (10 calibers)
Crew	12
Shell	700 lb (320 kg) 1,046 lb (474 kg)
Caliber	12 in (305 mm)
Breech	Slotted screw
Recoil	23 in (580 mm) (M1896MI) 24 in (610 mm) (M1896MII)
Elevation	45° (minimum for firing) 70° (maximum for firing)
Traverse	360°
Rate of fire	1 round per minute (normal) 1.3 rounds per minute (maximum)
Muzzle velocity	1,500 ft/s (460 m/s) (700-lb shell) 1,050 ft/s (320 m/s) (1,046-lb shell)
Effective firing range	14,610 yd (13,360 m) (maximum 700-lb shell) 2,400 yd (2,200 m) (minimum 1,046-lb shell)

The coast defense mortar was a massive weapon of 12-inch (305 mm) caliber emplaced during the 1890s and early 20th century to defend US harbors from seaborne attack. In 1886 when the Endicott Board set forth its plan for upgrading the coast defenses of the United States, it relied primarily on mortars, not guns, to defend American harbors. Over the years, provision was made for fortifications that would mount some 476 of these weapons, although not all of these tubes were installed. Ninety-one of these weapons were remounted as railway artillery in 1918-1919, but this was too late to see action in World War I. The railway mortars were only deployed in small quantities, and none overseas. The fixed mortars in the Philippines saw action in the Japanese invasion in World War II. All of the fixed mortars (except four) in the United States were scrapped by 1944, as new weapons replaced them, and the railway mortars were scrapped after the war. Today, the only remaining mortars of this type in the 50 states are four at Battery Laidley, part of Fort Desoto near St. Petersburg, Florida, but the remains of coast defense mortar emplacements can be seen at many former Coast Artillery forts across the United States and its former territories. Additional 12-inch mortars and other large-caliber weapons remain in the Philippines.

The M1890 M1 (Model of 1890, Modification 1) 12-inch mortar was one of the most powerful coast artillery pieces of its era, and was the most common type emplaced to guard U.S. harbors.

This mortar and other models, the M1886, M1908, and M1912, usually fired deck-piercing (also called armor-piercing) shells. These weighed from 700 to 1,046 pounds (318 to 474 kg) and had heavy, hardened steel caps, designed to pierce a ship's deck armor before the shell exploded. These mortars, firing the half-ton shells at an elevation of 45 degrees, had a range of 12,019 yards (10,990 m) (about 7 miles).

The deck-piercing shells were usually the ammunition of choice, because even the heaviest battleships of the 1890-1920 period were relatively lightly armored on the tops of their main decks, so a plunging half-ton shell could inflict crippling damage on one of them. Early on (from about 1890 to 1915), coast defense mortars were also supplied with so-called "torpedo shells" weighing 800 or 1,000 pounds (360 or 450 kg). (see illustration at right, below). These were thin-walled shells roughly 5 feet (1.5 m) in length that carried explosive charges of about 130 pounds (59 kg) and were meant to detonate upon contact with the deck of a ship, scattering fragments among the crew.