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Mortar, 81mm M1 Muzzle-loading 81mm Mortar (1935)

Authored By Silver Fox | Last Updated: 4/20/2011

The Mortar, 81mm M1, was another French-inspired weapon that went on to see successful combat actions with American forces in World War 2, Korea and Vietnam.

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The American M1 81mm Mortar, like the upcoming M2 60mm Mortar, was based on a French design by Edgar William Brandt (1880-1960). The M1 was a derivative of the French mle 27/31 system (itself an improved form of the Stokes Trench Mortar of World War 1 fame) and slightly altered to suit American needs. The M1 became the standard American battalion mortar of World War 2 and saw action in the Korean War as well as the Vietnam War until it was eventually replaced by the M29 - a lighter 81mm system with a longer reach.

As a weapons engineer, Frenchman Brandt was responsible for a slew of advancements in the field of mortars and projectiles that led to the development of varying 60mm, 81mm and 120mm systems while at the same time furthering HEAT rifle grenade and HEAT-warhead anti-tank weaponry technology. Brandt's designs were heavily copied throughout World War 2 and beyond, making them commonplace throughout the globe in the years following.

The US Army had already garnered experience in the use of the 3" Mk I Stokes Trench Mortar during World War 1. The type stayed in circulation in the post-war US Army though several attempts to find a replacement ultimately came to naught. As a result, the US Army decided to work instead on bettering ammunition. In 1931, the US Army acquired four of the Brandt 81mm prototypes, and, after some slight modifications to suit American use and production methods, the mortar appeared formally as the "81mm Mortar, M1" in Army nomenclature. License production was handled by A.B. Farquhar Company, Pullman-Standard Car Manufacturing and Watervliet Arsenal.

Like the "lighter" M2 system, the M1 was made up of three main components - the firing tube, bipod and baseplate. When completely assembled, the M1 weighed in at 136lbs. Weight was distributed as follows: the tube made up 44.5lbs while the mount was 46.5lbs. The base plate itself was 45lbs. Overall length of the system measured in at 3 feet, 9.5 inches. Muzzle velocity was rated at 700 feet per second out of the smoothbore firing tube. A sustained rate-of-fire of 18 rounds per minute was possible, with the operator loading the M1 by dropping the prepared projectile into the muzzle. A firing pin at the base of the firing tube activated the projectile's primer and ignition cartridge (the projectile was dropped down the tube "fuze-end" first) and the corresponding action launched the round at the predetermined desired angle. The operator(s) need only to protect themselves after the projectile was dropped in the tube. This allowed for an excellent sustained rate-of-fire - a maximum rate-of-fire of 30- to -35 rounds per minute was achievable. The M1 maintained a minimum range of 200 yards and a maximum range of 3,300 yards. Elevation was +40 to +85 with a traverse of 14-degrees. The operator utilized an M4 collimator sight (same as on the M2 60mm derivative) fitted to the bipod for accuracy calculations and adjustments.

The M1 could utilize a variety of ammunition types beginning with the M43A1 Light HE (High-Explosive) round. The 6.87lb M43A1 maintained a fragmentation radius of 25 yards and featured a surface detonating fuze. The 10.6lb M45 and M45B1 Heavy HE rounds greater explosive punch at the expense of range (2,558 yards), this working from a delayed fuze. The 15lb M56 was another Heavy HE round of even greater explosive firepower with a more limited range of 1,300 yards. The fuze on these particular projectiles was adjustable as needed. Shells were stabilized along their flight path via fixed fins at their rear sections to compensate for the M1's use of a smoothbore firing tube (i.e. no rifling for inherent course trajectory).

Besides the conventional explosive rounds, the M1 could also make use of the 10.7lb M57 FS white smoke round out to 2,470 yards or the 10.74lb M57 WP (White Phosphorous), this also ranged out to 2,470 yards. The White phosphorous round was also equally adept at attacking infantry as an incendiary munition. The M301 was a useful illuminating round with an adjustable fuse and 60-second burn duration and deployable parachute, the latter helping to retard the projectile's fall.

Transport of the heavy M1 was solved through the use of a two-man hand cart designated simply as the "Hand Cart M6A1". This allowed the least possible amount of crew to move the mortar system about, allowing transport of the weapon into defensive or offensive positions as required with relatively little pain. Another option was to have the system towed via mule by way of a specially-devised harness. Despite its inherent weight drawback of the M1 kit, the firepower of the mortar offset any negatives especially when supporting artillery would not be made available in a given operation. A well-trained and experienced mortar team could engage targets in defilade, trenches, ravines or on slopes and, as such, proved lethal against enemies in both the European and Pacific theaters of war.

For another method of transportation, the M1 could most effectively be driven around on the back of an M3 Halftrack. These half-tank/half-truck "mutt" vehicles held the inglorious task of transporting just about anything under the wartime sky. From the rear of the M3, the M1 could be fired without the need for the crew and weapon to disembark from the vehicle, affording a certain level of "hit-and-run" tactical advantage. At any rate, this method of fielding the M1 proved most useful to any company commander.

A shortened version of the M1 was unveiled as the T27 "Universal" but this only appeared in limited numbers and was never ordered for mass production. A special tube extension system was also devised for the base M1, though this was hardly used in practice.

The M1 81mm Mortar was complimented in the field by the lighter M2 60mm Mortar beginning in 1940. The M2 was a more "portable" device that allowed for efficient close-support indirect fire and could operate closer to the actions of frontline infantry, its portability being a major plus.

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Specifications for the
Mortar, 81mm M1
Muzzle-loading 81mm Mortar


Country of Origin: United States
Manufacturer: A.B. Farquhar Company; Pullman-Standard Car Manufacturing; Watervliet Arsenal - USA
Initial Year of Service: 1935


Overall Length: 944mm (37.17in)
Barrel Length: 0.00mm (0.00in)
Weight (Empty): 299.83lbs (136.00kg)


Caliber: 81mm
Action: Muzzle Loading; Primer Impact
Feed: Single-Shot
Muzzle Velocity: 700ft/sec (213m/sec)
Rate-of-Fire: 18 rounds per minute
Range: 9,900ft (3,018m; 3,300yds)
Sights: M4 collimater sight


Variants:
Mortar, 81mm M1 - Base Series Designation


T27 "Universal" - Short-lived, shortened form of the M1; never selected for mass production.


Operators:
United States