The M18A1 Claymore is a directional anti-personnel mine used by the U.S. military. It was named after the large Scottish sword by its inventor, Norman A. MacLeod. The Claymore fires shrapnel, in the form of steel balls, out to about 100 meters across a 60° arc in front of the device. It is used primarily in ambushes and as an anti-infiltration device against enemy infantry. It is also of some use against soft-skinned vehicles.
Numerous licensed and unlicensed copies of the mine are produced in numerous countries. Examples are the former Soviet Union models MON-50, MON-90, MON-100, MON-200; MRUD (Serbia); No. 6 (Israel); MAPED F1 (France); Mini MS-803 (South Africa), and more.
When the M18A1 is detonated, the explosion drives the matrix of 700 spherical fragments out of the mine at a velocity of 3995 feet per second (1,200 m/s) , at the same time breaking the matrix into individual fragments. The spherical steel balls are projected in a 60° fan-shaped pattern that is two meters high (6 ft, 8 in) and 50 meters (165 ft) wide at a range of 50 meters (165 ft). The force of the explosion deforms the relatively soft steel fragments into a shape similar to a .22 rimfire projectile . These fragments are moderately effective up to a range of 100 meters (328 ft), with a hit probability of around 10% on a prone man-sized target (0.12 square meters). The fragments can travel up to 250 meters (820 ft) forward of the weapon. The optimum effective range is 50 meters (165 ft), at which the optimal balance is achieved between lethality and area coverage with a hit probability of 30% on a man-sized target. The weapon and all its accessories are carried in the M7 bandolier. An instruction sheet for the weapon is attached to the inside cover of the bandolier.
The Claymore mine is typically deployed in one of three modes: Controlled, Uncontrolled, or Time-delayed.Controlled Mode (also known as Command Detonation)
Following the massed Chinese attacks during the Korean War, Canada and the United States began to develop projects to counter them. Canada fielded a weapon called the "Phoenix" landmine that used the Misznay-Schardin effect to project a spray of steel cubes towards the enemy. The cubes were embedded in five pounds of Composition B explosive. It was too large to be a practical infantry weapon and was relatively ineffective with a maximum effective range of only 20 to 30 yards (20 to 30 meters).
Around 1952 Norman MacLeod at his company the Explosive Research Corporation began working on the concept of a small directional mine for use by infantry. It is not clear if Picatinny took the concept from this Canadian weapon and asked Norman MacLeod to develop it; or if he came up with the idea independently and presented it to them. MacLeod came up a design, the T-48 that was broadly similar to the final M18A1, although it lacked a number of the design details that made the M18A1 effective. It was accepted into Army service as the M18 Claymore and approximately 10,000 were produced. It was used in small numbers in Vietnam from around 1961, but it wasn't until the arrival of the improved M18A1 that it became a significant weapon.
The M18 was 235 millimeters long and 83 millimeters high with a plastic case with three folding spike legs on the bottom. An electrical blasting cap for triggering the mine was inserted through a small hole in the side. Internally the mine consisted of a layer of 340 grams of C-3 explosive (the forerunner of C-4 explosive) in front of which was laid an array of steel cubes. In total the mine weighed about 1.1 kilograms, and could be fitted with an optional peep sight for aiming. . It lacked the later version's iconic "FRONT TOWARD ENEMY" marking. The mine was planted in the ground using its three sharp legs and was aimed in the direction of enemy approach and then fitted with an electrical blasting cap. The mine was then triggered from a safe position, preferably to the side and rear.
The mine was barely more than a prototype and was not considered a "reliable casualty producer" with an effective range like the Phoenix of only 90 feet (30 m).
MacLeod applied for a patent for the mine on 18 January 1956 and was granted it in February 1961. The patent was later the source of a civil court case between MacLeod, the Army and Aerojet who proceeded to develop the design further. MacLeod's case collapsed when photographs of the German Trenchmine prototype were produced as evidence of prior art.
In 1954 Picatinny Arsenal issued a request for proposals (RFP) aimed at improving the M18 into a more effective weapon. Guy C. Throner working at Aerojet had independently come up with a design for a Claymore-like mine in the early 1950s. Seeing the RFP and working with Don Kennedy they submitted a 30 page proposal to Picatinny and were awarded a $375,000 development contract to improve the design. Picatinny criteria for the weapon were as follows
The requirement for kinetic energy came from the fact that 58 foot pounds is the amount of kinetic energy required to deliver a potentially lethal injury . Given the requirements of weight, and fragment density required this dictated using approximately 700 fragments, and being able to aim the mine with an accuracy of around two feet (0.6 m) at the center of the target zone. The team at Aerojet were given access to all previous research into the directional mine, including the M18 and the Phoenix mine, as well as German research. Dr. John Bledsoe led the initial project.
In testing it became apparent that the original M18 mine fell far short of the Picatinny requirements. One of the first improvements was to replace the steel cubes with 7/32 inch hardened 52100 alloy ball bearings. These performed poorly for two reasons, the hardened steel balls, spalled into fragments when hit by the shock of the military explosive, the fragments were neither aerodynamic nor large enough to perform effectively. Additionally the blast "leaked" between the balls, actually reducing their maximum velocity.
A second problem to be addressed was the optimum curvature of the mine. This was determined experimentally by Bledsoe, through a large number of test firings. Bledsoe left the project to work at the Rheem corporation, and another engineer, William Kincheloe came onto the project.
Kincheloe immediately came up with the suggestion to use softer 1/8 inch steel "gingle" balls that were used in the foundry process. The softer balls did not spall when struck by the shock from the explosive, instead they deformed into a useful aerodynamic shape similar to a .22 rimfire projectile. Using a homemade Chronograph these were clocked at per second (1,150 meters per second). The second optimization was to use a poured plastic matrix to briefly contain the blast from the explosive, so that more of the blast energy was converted into projectile velocity. After a number of weeks of experimentation they finally settled on Devcon-S steel filled epoxy to hold the steel balls against the explosive. With this in place the velocity improved to per second (1,200 meters per second).
There were still a number of technical challenges to overcome, including the development of a case that would be able to contain the corrosive C-3 explosive, and be tough enough to withstand months of field handling in wide temperature ranges. Using soluble dyes to test various plastics for leaks, they found a suitable plastic called Durex 1661 1/2 which could be easily molded into a case.
By the Spring of 1956 they had a near final design, which was awarded a preproduction contract for 1,000 M18A1 claymores designated T-48E1 during testing.
The initial versions of the mine used two pairs of spindly wire legs produced from number 9 wire. Later when production was ramped up the design was changed to flat steel scissor, folding-type legs.
Early pre-production mines were triggered using a battery pack that was used with the M18, however this was undesirable for a number of reasons. Bill Kincheloe came up with the idea of using a "Tiny Tim" toggle generator used with a number of Navy Rockets. Originally an aluminium box was used to hold the generator. Later a Philadelphia company "Molded Plastic Insulation Company" took over the manufacture of the firing device for the first large scale production run producing a plastic device.
The sighting for the device was also originally intended to be a cheap pentaprism device, that would allow the user to look down from above and see the sight picture. After finding a suitably low cost device, it was found that fumes from either the C-3 explosive or the cement used to adhere the sight to the top of the mine corroded the plastic mirrors, rendering it unusable. In the end simple peep sights were adopted, which was later replaced by a knife blade sight.
Testing conducted with the mine concluded that the mine was effective out to approximately , being capable of hitting 10 % of the attacking force. At this increased to 30 %. The development project completed, the Aerojet team sent the project back to Picatinny, where it was bid out to various component suppliers. It was type standardized as the M18A1 in 1960 seeing first active service in Vietnam in spring or early summer 1966.
Minor modifications were made to the mine during its service. A layer of tinfoil has been added to the design, fixed between the fragmentation matrix and the explosive. This serves to slightly improve the fragment velocity, and to chemically protect the steel fragments from the corrosive explosive compound. Additionally a ferrite choke is used to prevent RF signals and lightning triggering the mines.