M18 Claymore mine

The M18A1 Claymore is a directional anti-personnel mine used by the U.S. military. Its inventor, Norman MacLeod, named the mine after a large Scottish medieval sword. Unlike a conventional land mine, the Claymore is command-detonated and directional, meaning it is fired by remote-control and shoots a pattern of metal balls into the kill zone like a shotgun.

The Claymore fires steel balls, out to about 100 m 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 used against unarmored vehicles.

Many countries have developed and use mines like the Claymore. 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).

Description
The M18A1 Claymore mine has 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 range. The case has the words "Front Toward Enemy" embossed on the front 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 in steel balls (about as big as #4 birdshot) set into an epoxy resin.

When the M18A1 is detonated, the explosion drives the matrix forward, out of the mine at a velocity of 1200 m/s, at the same time breaking it into individual fragments. The steel balls are projected in a 60° fan-shaped pattern that is 6.5 feet high and 50 m wide at a range of 50 m. The force of the explosion deforms the relatively soft steel balls into a shape similar to a .22 rimfire projectile. These fragments are moderately effective up to a range of 100 m, with a hit probability of around 10% on a prone man-sized 1.3 sqft target. The fragments can travel up to 250 m. The optimum effective range is 50 m, 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 an M7 bandolier. The mine is detonated as the enemy approaches the killing zone. Controlled detonation may be accomplished by use of either an electrical or non-electrical 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 detonate several mines.

The mine can be detonated by any mechanism that activates the blasting cap. There are field-expedient methods of detonating the mine by tripwire, or by a timer, but these are rarely used.

Development
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.

Norman MacLeod and Explosive Research Corporation
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, which used the Misznay-Schardin effect to project a spray of 0.25 in 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 (about 20 to 30 meters).

Around 1952 Norman MacLeod, at his company the Explosive Research Corporation, began working on a small directional mine for use by infantry. It is not clear if the United States Picatinny Arsenal took the concept from the Canadian weapon and asked Norman MacLeod to develop it; or if he developed the design independently and presented it to them. MacLeod designed a weapon called the T-48; broadly similar to the final M18A1, it lacked a number of the design details that made the M18A1 effective.

Through Picatinny, the United States Army accepted the weapon into service as the M18 Claymore and approximately 10,000 were produced. It was used in small numbers in Vietnam from around 1961. It was not until the improved M18A1 was developed that the Claymore became a significant weapon.

The M18 was 9.25 in long and 3.27 in high, held in 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 oz of C-3 explosive (the forerunner of C-4 explosive) in front of which was laid an array of 0.25 in steel cubes. In total the mine weighed about 2.43 lb, 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 aimed in the direction of enemy approach; at that point, it was fitted with an electrical blasting cap. The mine was 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"; it had 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 subject of a civil court case between MacLeod, the Army, and Aerojet, which further developed the Claymore design. MacLeod's case collapsed when photographs of the German Trenchmine prototype were produced as evidence of prior art.

Throner, Kennedy, Bledsoe, and Kincheloe at Aerojet
In 1954 Picatinny Arsenal issued a request for proposals (RFP) to improve the M18 as a more effective weapon. At Aerojet in the early 1950s, Guy C. Throner had independently come up with a design for a Claymore-like mine. He worked with Don Kennedy and the two men submitted a 30-page proposal in response to Picatinny's RFP. They were awarded a $375,000 development contract to improve the Claymore design. The 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 sqft 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 4000 ft per second providing 58 foot-pounds (79 joules) of kinetic energy delivered to the target.

The requirement for kinetic energy was based on the fact that 58 foot pounds is required to deliver a potentially lethal injury. Given the requirements of weight and fragment density, approximately 700 fragments were needed, with the ability 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 directional mines, including the M18 and the Phoenix, as well as German research. Dr. John Bledsoe led the initial project.

The original M18 mine fell far short of Picatinny's requirements. One of the first improvements was to replace the steel cubes with 7/32 in 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 was the curvature of the mine. This was determined experimentally by Bledsoe, through a large number of test firings. After Bledsoe left the project to work at the Rheem corporation, William Kincheloe, another engineer, came onto the Claymore project.

Kincheloe immediately suggested using softer 1/8 in steel "gingle" balls, which were used in the foundry process. They did not spall from the shock of the explosive, but deformed into a useful aerodynamic shape similar to a .22 rimfire projectile. Using a homemade chronograph, the engineers clocked the balls at 3775 ft per second. The second change 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, the engineers settled on Devcon-S steel-filled epoxy to hold the balls in place. With this change, the velocity improved to 3995 ft per second.

Technical challenges to overcome included developing a case to contain the corrosive C-3 explosive that would be durable 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, aerojet had a near-final design. It was awarded a pre-production 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, which had been used with the M18. This was found to be undesirable for a number of reasons. Bill Kincheloe came up with the idea of using a "Tiny Tim" toggle generator, of the type 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.

The sighting for the device was originally intended to be a cheap pentaprism device, which would allow the user to look down from above and see the sight picture. After locating a suitably low-cost device, the engineers 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. they adopted simple peep sights, which were later replaced by a knife blade sight.

Testing concluded that the mine was effective out to approximately 110 yd, being capable of hitting 10% of the attacking force. At 55 yd, this increased to 30%. The development project completed, the Aerojet team sent the project back to Picatinny. The Arsenal bid it out to various component suppliers. In 1960 it was type standardized as the M18A1. It was first used in Vietnam in the 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.

National copies
A number of licensed and unlicensed copies of the mine have been produced.
 * 🇨🇱 Chile: M18
 * 🇨🇳 China: Type 66
 * 🇫🇮 Finland: Viuhkapanos (Fan Charge) VP 88 and "heavier" VP 84
 * 🇭🇺 Hungary: IHR-60
 * 🇮🇹 Italy: VS-DAFM 7
 * 🇵🇰 Pakistan: P5 Mk1
 * 🇵🇭 Philippines: M18A2
 * 🇵🇱 Poland: M18 Claymore
 * 🇷🇺 Russia: MON-50
 * 🇿🇦 South Africa: Shrapnel mine No 2
 * 🇰🇷 South Korea:
 * K440, slightly smaller than the Claymore with 770 fragments.
 * KM18A1


 * 🇸🇪 Sweden:
 * FFV-013
 * Försvarsladdning 21
 * LI-12/Truppmina 12


 * 🇹🇷 Turkey: M18 AP Mine
 * 🇺🇸 United States: Arms Tech MM-1 "Minimore", a smaller variant conceived for Special Forces use
 * Vietnam: MDH-C40
 * Yugoslavia: MRUD (Mina Rasprskavajuća Usmerenog Dejstva)