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. Unlike a conventional land mine, the Claymore is command-detonated and directional, meaning it is fired by remote-control, shooting a pattern of metal balls into the kill zone like a shotgun.
The M18A1 Claymore mine with the plastic trigger and detonator wire.
The Claymore fires steel balls, out to about 100 meters within 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 unarmored vehicles.
Many countries use mines like the Claymore, some of which seem to be copies of the American design. Examples include former Soviet Union models MON-50, MON-90, MON-100, MON-200, MRUD (Serbia), MAPED F1 (France), and Mini MS-803 (South Africa).
The M18A1 Claymore mine consists of a horizontally convex green plastic case (inert training versions are blue). The shape was developed through experimentation to deliver the optimum distribution of fragments at 50 m (55 yd) range. The case has the words "Front Toward Enemy" embossed on the front surface of the mine. A simple open sight on the top surface allows for aiming the mine. Two pairs of scissor legs attached to the bottom support the mine and allow it to be aimed vertically. On both sides of the sight are fuse wells set at 45 degrees.
Internally the mine contains a layer of C-4 explosive behind a matrix of about seven hundred 1⁄8-inch-diameter (3.2 mm) steel balls (about as big as #4 birdshot) set into an epoxy resin.
When the M18A1 is detonated, the explosion drives the spheres out of the mine at a velocity of 1,200 m/s (3,937 ft/s), at the same time breaking the matrix into individual fragments. The steel balls are projected in a 60° fan-shaped pattern that is 6.5 feet high and 50 m (55 yd) wide at a range of 50 m (55 yd). 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 m (110 yd), with a hit probability of around 10% on a prone man-sized 1.3-square-foot (0.12 m2) target. The fragments can travel up to 250 m (270 yd). The optimum effective range is 50 m (55 yd), 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 a bandolier. An instruction sheet for the weapon is sewn inside the cover of the bandolier.
A Marine emplaces a Claymore mine.
Ideally, the mine is detonated as the enemy approaches the killing zone 20 to 30 m (22 to 33 yd), where maximum casualties can be inflicted. Controlled detonation may be accomplished by use of either an electrical or nonelectrical firing system. When mines are employed in the controlled role, they are treated as individual weapons and are reported in the unit fire plan. They are not reported as mines; however, the emplacing unit must ensure that the mines are removed, detonated, or turned over to a relieving unit. The M57 Firing Device (colloquially referred to as the "clacker") is included with each mine. When the mines are daisy chained together, one firing device can initiate several mines.
The mine can be detonated by any mechanism that activates the blasting cap. Field-expedient methods of detonating the mine by tripwire, or even by a timer, exist, but are rarely used.
The development of the M18A1 mine dates back to work done during World War II. The Misznay-Schardin effect was independently discovered during World War II by Misznay, a Hungarian, and Dr. Hubert Schardin, a German. When a sheet of explosive detonates in contact with a heavy backing surface (for example, a metal plate), the resulting blast is primarily directed away from the surface in a single direction. Schardin spent some time developing the discovery as a side-attack anti-tank weapon, but development was incomplete at the end of the war. Schardin also spent time researching a "Trench mine" that used a directional fragmentation effect.
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 0.25-inch (6.4 mm) 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).
Images from the 1956 Macleod patent.
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 with 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 was not until the arrival of the improved M18A1 that it became a significant weapon.
The original M18 Claymore mine. Note the detonator inserted into the side.
The M18 was 9.25-inch (235 mm) long and 3.27-inch (83 mm) 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 12-ounce (340 g) of C-3 explosive (the forerunner of C-4 explosive) in front of which was laid an array of 0.25-inch (6.4 mm) steel cubes. In total the mine weighed about 2.43-pound (1.10 kg), 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
- It must weigh less than 3.5 pounds (1.6 kilograms)
- It must throw enough fragments so that at a range of 55 yards (50 m) it achieves a 100 percent strike rate on a 1.3 square feet (0.12 m2) target (man sized)
- The fragment area must not be more than 8 feet (2.4 m) high and no more than 60 degrees wide
- Fragments must have a velocity of 4,000 feet (1,200 m) per second providing 58 foot-pounds (79 joules) of kinetic energy delivered to the target.
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.
The original M18 mine fell far short of requirements. One of the first improvements was to replace the steel cubes with 7⁄32-inch (5.6 mm) hardened 52100 alloy ball bearings. These performed poorly for two reasons. First, the hardened steel balls spalled into fragments when hit by the shock of the explosion; the fragments were neither aerodynamic enough nor large enough to perform effectively. Additionally the blast "leaked" between the balls, reducing their velocity.
A second problem to be addressed was the 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 (3.2 mm) 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 3,775 feet (1,151 m) 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 experiments they settled on Devcon-S steel filled epoxy to hold the balls in place. With this improvement, the velocity improved to 3,995 feet (1,218 m) 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 dyes to test various plastics for leaks, they found a suitable plastic called Durex 1661½ 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 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 aluminum 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.
A Chinese Type 66 claymore mine.
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 them unusable. In the end simple peep sights were adopted, which was later replaced by a knife blade sight.
Testing concluded that the mine was effective out to approximately 110 yards (100 m), being capable of hitting 10% of the attacking force. At 55 yards (50 m) 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 was added between the fragmentation matrix and the explosive. This slightly improves the fragment velocity, and protects the steel fragments from the corrosive explosive. A ferrite choke was added to prevent RF signals and lightning from triggering the mine.
The use of this mine is permitted by the Ottawa Mine Ban Treaty as it is most often command-detonated.