12 - inch coast defense mortar The coast defense mortar was a massive weapon of 12 - inch caliber emplaced during the 1890s and early 20th century to defend U . S . 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 . Today , the only remaining mortars of this type in the 50 states exist at Battery Laidley , part of Fort Desoto in Florida , but the remains of coast defense mortar emplacements can be seen at many Coast Artillery forts across the United States and its former territories . The weapons . The 1890 M1 ( Model 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 its later model , the M1908 usually fired deck - piercing ( also called armor - piercing ) shells . These weighed from 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 ( 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 . ( see illustration at right , below ) . These were thin - walled shells roughly in length that carried explosive charges of about and were meant to detonate upon contact with the deck of a ship , scattering fragments among the crew . The M1890M1 mortar was most often installed on an M1896 carriage ( as shown here in the images at top left and right ) . The mortar and its carriage weighed a total of 78 . 5 tons . 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 . A ring marked in degrees of azimuth ran around the mortar , just outside the inner steel circle or " racer " that carried the carriage , and a soldier read a pointer on the racer to aim the mortar in direction . The tube was raised or lowered ( in elevation ) by twin geared wheels with long spokes ( resembling ship ' s wheels ) that were located on either side of the carriage . The breech could be rapidly depressed to an almost level position for loading and then be quickly elevated for firing . 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 . The desired range for the mortar was specified in terms of zones . The smallest zone ( shortest range ) was Zone 1 , and the largest ( longest range ) was Zone 9 . With the so - called " aliquot charge " ( shown in the photo at right ) , up to 9 equal - sized , disk - shaped bags of powder ( each about thick and containing of powder ) could be attached to a 10th ( or " base " ) bag , by means of cloth binding straps that were sewn to the base bag . Often the base bag was painted red , indicating that the powder assembly was to be loaded into the breech " red end last , " so that it bumped up against the closing breech block ( or faced the gunner ) . The red base bag also contained a small charge of black powder as an igniter . When the breech was closed , a detonator was inserted through the breech block and contacted the igniter , ready to set off the full powder charge . How the mortar emplacements were designed . The earliest coast defense mortar batteries of the modern era were designed ( starting in about 1892 ) as so - called " Abbot Quads " . Named for Gen . Henry Larcom Abbot , the Army engineer who invented the design , Abbot Quads were rectangular configurations of four rectangular mortar pits , with each pit designed to mount four of the huge mortars . A plan for an Abbot Quad mortar battery is shown at left . 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 . It was argued that targeting each mortar individually would not produce many more hits than salvo firing , since early fire control procedures and equipment were often error - prone . Furthermore , proponents of salvo firing pointed out that it made for easier command and control ( particularly under battle conditions ) , since all mortars in all pits of a battery could be given the same firing data . Early battery designs often contained one central " firing room " from which cables ran out to the various mortar pits , enabling electrical firing of all weapons simultaneously . However , the early Abbot Quad designs featured very small mortar pits . Often , four mortars were mounted in a pit that was only about in size . Four circular areas about in diameter would fit into one of these early pits , producing a very crowded situation ( given that two adjacent M1890 mortars mounted in such a pit would have their muzzles almost touching if they were traversed to face each other ) . A photo of such an early " crowded pit " is shown at left . When a mortar battery was fully manned , formal guidelines called for a pit containing just two mortars ( see photo at right ) to be manned by a pit commander , two mortar squads of 17 enlisted men each , and an ammunition squad of 16 enlisted men . One of the last versions of the manual for the 12 - inch mortar gives details on how it was to be crewed and fired . After about 1905 , reliance on the Abbot Quad design declined . Some artillery officers argued strongly 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 . They also argued that the smaller , cramped mortar pits of the early Abbot Quad battery designs were simply too crowded for efficient operations , with mortar crews for different tubes constantly getting in each other ' s way . As fire control methods improved and the network of base end station s was extended , from about 1905 on , individual aiming of mortars could be more accurate . At the same time , designs for new pits often specified only two mortars per pit , and many forts constructed their mortar pits side - by - side . Many 4 - mortar pits were " depopulated , " and some of their tubes were sent away to equip newer batteries . The Abbot Quad designs passed from favor . In the 1930s , the US Army tested 12 - inch mortars that were mounted on special railway cars and could be moved about in a time of crisis . The following footnoted article shows the firing of a standard M1890 mortar encased in a special bracket and mounted on a special flatcar ( with outriggers ) , that was known as the M1918 Carriage ( Railway ) . The combination apparently weighed 88 tons . Firing the mortars . The Coast Defense Study Group ( CDSG ) has found and posted this unique film clip that dates from around 1915 and shows ( during its first 3 minutes or so ) 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 film clip clearly illustrates how congested one of the old - style pits would have become if used to fire four mortars simultaneously or nearly so . The film shows the heavy shells ( on shell carts ) being wheeled up to the breeches of the mortars and being rammed home , the powder bags being tossed into the breeches after them , the crew clearing the immediate area while the chief of the breech raises his arm to indicate " ready to fire , " and the mortars being fired electrically ( from outside the area pictured ) . The tubes are then depressed , crewmen rush back in to swab out the tubes , the tubes are elevated , and crewmen back off ( " taking cover " ) , firing occurs , and the process repeats itself . The shock wave from firing just one of these huge mortars , particularly if it was fired within one of the smaller , old - style pits was often so strong that it destroyed sensitive equipment mounted near the pit , knocked doors off of nearby magazines and barracks buildings , and broke windows in neighborhoods that were near the mortar batteries . The thundering crash of four of these mortars being fired simultaneously in a pit must have been overwhelming . Fire control . The images here show another feature of the mortar pits — the data booth . Personnel in this small space , with its tapered viewing slits , received the coordinates ( azimuth and elevation ) that had been calculated via plotting board by the battery ' s Range Unit as the firing coordinates for the mortars in order for them to hit their targets . These booths were either small , free - standing structures , about square and tall ( as shown here in the topmost photos ) , or were built into one of the walls of the mortar pit itself ( lower photo at right ) . The reconstructed data board in the photo at left hangs off the side of the data booth and is used to post the firing coordinates for mortars # 3 and # 4 in its pit ( evidently the only two mortars there ) . The " Zone " number posted on the board refers to the size of the powder charge to be loaded for the coming shot ( as discussed above ) . The top photo at right shows a decomposed set of slats ( which likely used to have slates attached to them ) that could have firing data chalked onto them and then be slid out of the data booth so they could be seen by the mortar crews in the pit . Since crews of the mortars could not see their targets , they were especially dependent on the overall fire control system , with its base end station s and plotting room , to locate and pinpoint targets for them to hit .