Following the massed Chinese attacks during the Korean War, the US Army developed the German design into an anti-personnel device that would fire hundreds of small ball-bearings in one shot. The rear of the device was a steel sheet covered with the explosive, and in front was an air space packed with the shot. When the explosive detonates the majority of the force is directed forward, sending the balls out the front at high velocity. Claymores were not buried like mines but were placed above ground pointed towards the likely location of the enemy.
As developed for the Vietnam War, the Claymore had an plastic olive casing with the words "Front Toward Enemy" on it. It was 21.5 centimeters (eight inches) long, 8 centimeters (three inches) high, and 3.5 centimeters (inch and a half) deep with two sets of little legs. Inside is 650 grams of plastic explosive and 700 steel ball-bearings. The Claymore is normally fired on command using a hand held electrical initiator provided with the weapon. However because of its simple design, it could also be fired in a variety of other ways, including electric detonator, primacord, motion sensor, or even converted into a landmine or booby trap by addition of a booby trap switch and trip wire. (This latter procedure is illegal in many countries.)
The M18A1 was standardized in 1960, and replaced the M18 antipersonnel weapon. Both weapons are similar in appearance and functioning. The M18A1 Claymore is equipped with a fixed plastic slit-type sight (knife-edge sight on later model), adjustable legs, and two detonator wells. The weapon and all its accessories are carried in the M7 bandoleer. An instruction sheet for the M18A1 is attached to the inside cover of the bandoleer.
When detonated, the M18A1 Claymore delivers spherical steel fragments over a 60° fan-shaped pattern that is two meters high and 50 meters wide at a range of 50 meters. These fragments are moderately effective up to a range of 100 meters and can travel up to 250 meters forward of the mine. The optimum effective range (the range at which the most desirable balance is achieved between lethality and area coverage) is 50 meters.