The 12-inch coast defense mortar was a weapon of 12-inch (305 mm) caliber emplaced during the 1890s and early 20th century to defend US harbors from seaborne attack.

[note 1] In 1886, when the Endicott Board set forth its initial plan for upgrading the coast defenses of the United States, it relied primarily on mortars, not guns, to defend American harbors.

[3] The M1890M1 (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.

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.

[note 4][5] 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.

The carriage was geared to enable it to be turned (in azimuth) by means of a traversing crank with two handles, located on the right side of the piece.

[7] In addition to the elevation of its tube, the factor that determined the mortar's range was the size of the powder charge that was loaded into its breech, following the shell.

Third, the circuit switch (usually located on the wall of the pit near the data booth) leading to the individual mortar had to be thrown into the closed (firing) position.

The earliest coast defense mortar batteries of the modern era were designed as so-called "Abbot Quads".

The earthen prototype for these was built in the 1870s at Fort Totten in New York City, and construction of the first operational Abbot Quads commenced circa 1892.

[9] The idea behind the Abbot Quad was to have all 16 mortars in the four pits fire at once, producing a shotgun-like salvo of plunging shells optimally dispersed to destroy an enemy ship.

[13] Some artillery officers[note 10][1] argued that salvo firing was inherently wasteful, and that a much better hit ratio could be achieved by aiming each mortar individually against a specified target.

This film clip that dates from around 1915 shows a firing drill on the 12-inch mortars of Battery Howe, part of the Harbor Defenses of San Francisco.

Although the battery shown is a linear one, the firing drill is similar to what would have taken place in a square or rectangular pit at Fort Banks.

The images here show another feature of the mortar pits—the data booth, part of the Coast Artillery fire control system.

After the American entry into World War I on 6 April 1917, the US Army considered converting coast artillery weapons to railway mounts for use on the Western Front.

However, as with other US pre-1940 fortifications, most of which were designed well before the airplane was invented, the open-topped emplacements left the mortars open to air and high-angle artillery attack.

Other 12-inch mortars were at Battery Koehler on Carabao Island (Fort Frank), which was used for ineffectual counter-battery fire against the Japanese Kondo Detachment artillery emplaced on the southern shore of Manila Bay,[21] and at Battery Craighill on Caballo Island (Fort Hughes), which fired at targets generated locally and from Corregidor.

The leading theories were that either the low rotational velocity of the shell or the small booster charge in the fuzes precluded detonation.