|Place of origin||United Kingdom|
|No. built||over 75 million|
|Variants||No. 5; No. 23 Mk I, II and III; No. 36 Mk I; No. 36M Mk I; No. 36 Mk II|
|Mass||765 g (1 lb 11.0 oz)|
|Length||95.2 mm (3.75 in)|
|Diameter||61 mm (2.4 in)|
|Percussion cap and time delay fuse: 7 seconds, later reduced to 4|
"Mills bomb" is the popular name for a series of British hand grenades. They were the first modern fragmentation grenades used by the British Army and saw widespread use in the First and Second World Wars.
William Mills, a hand grenade designer from Sunderland, patented, developed and manufactured the "Mills bomb" at the Mills Munitions Factory in Birmingham, England, in 1915. The Mills bomb was inspired by an earlier design by Belgian captain Leon Roland, who later engaged in a patent lawsuit. Col. Arthur Morrow, a New Zealand Wars officer, also believed aspects of his patent were incorporated into the Mills Bomb. The Mills bomb was adopted by the British Army as its standard hand grenade in 1915 as the No. 5.
The Mills bomb underwent numerous modifications. The No. 23 was a No. 5 with a rodded base plug which allowed it to be fired from a rifle. This concept evolved further with the No. 36, a variant with a detachable base plate for use with a rifle discharger cup. The final variation of the Mills bomb, the No. 36M, was specially designed and waterproofed with shellac for use in the hot climate of Mesopotamia in 1917 at first but remained in production for many years. By 1918 the No. 5 and No. 23 had been declared obsolete and the No. 36 (but not the 36M) followed in 1932.
The Mills was a classic design; a grooved cast iron "pineapple" with a central striker held by a close hand lever and secured with a pin. According to Mills's notes, the casing was grooved to make it easier to grip, not as an aid to fragmentation; and it has been shown that it does not shatter along the segmented lines. The Mills was a defensive grenade meant to be thrown from behind cover at a target in the open, wounding with fragmentation, as opposed to an offensive grenade, which does not fragment, relying on short-range blast effect to wound or stun the victim without endangering the thrower with fragments, which travel a much longer distance than blast. Despite the designations and their traits, "defensive" grenades were frequently used offensively and vice versa. A competent thrower could manage 49 ft (15 m) with reasonable accuracy, but the grenade could throw lethal fragments farther than this. The British Home Guard were instructed that the throwing range of the No. 36 was about 30 yd (27 m) with a danger area of about 100 yd (91 m).
At first the grenade was fitted with a seven-second fuse but in the Battle of France in 1940 this delay proved to be too long, giving defenders time to escape the explosion, or even to throw the grenade back. Therefore, the delay was reduced to four seconds. The British Army continued to use grenades with a seven second fuse for use in rifle mounted grenade projectors where the fuse time was required to allow the grenade to reach its full range.
The heavy segmented bodies of "pineapple" type grenades result in an unpredictable pattern of fragmentation. After the Second World War, Britain adopted grenades that contained segmented coiled wire in smooth metal casings. The No. 36M Mk.I remained the standard grenade of the British Armed Forces and was manufactured in the UK until 1972, when it was replaced by the L2 series. The 36M remained in service in some parts of the world such as India and Pakistan, where it was manufactured until the 2000s. Mills bombs were still being used in combat as recently as 2004, for example in the incident which killed US Marine Jason Dunham and wounded two of his comrades. The last major operator of the Mills bomb was india who only replaced it in August 2021 with a new Multi-Mode Hand Grenade (MMHG).
The Mills bomb was developed into a rifle grenade by attaching a metallic rod to its base. This rod-type rifle-grenade had an effective range of about 150 yards (140 m). The operating procedure was to insert the Mills bomb rod down the barrel of a standard rifle, put a special blank cartridge (Ballistite cartridge) in the rifle's chamber, place the rifle stock on the ground, then pull the Mills bomb's safety pin, releasing the safety lever and immediately fire the rifle. If the soldier did not launch the grenade quickly, the grenade's fuse would time out and explode. The British soon developed a simple cradle attached to the rifle's bayonet lug to hold the safety-lever in place and prevent accidental detonations. However, it was found that the repeated launching of rod-type grenades caused damage to the rifle's barrel, causing the middle to bulge out due to the prolonged pressure spike from driving the much heavier, larger projectile up the barrel (typically a much faster process with a normal bullet); a rifle cartridge rapidly burns up all the available powder, which fills the volume behind the bullet with extremely high pressure gases (tens of thousands of PSI), the pressure rising as the bullet moves up the barrel, peaking at some point before the bullet leaves the muzzle. With the much heavier grenade and rod, the cartridge had to accelerate a much heavier mass, which resulted in the powder burning up and the pressure peaking before the rod had got more than a part of the way up the barrel, putting peak pressure on sooner and sustaining it for longer.
The British subsequently developed a cup-type launcher to replace the rod-type rifle-grenade. In this design, a can-shaped launcher was attached to the muzzle of the rifle and a gas check disc was screwed onto the base of the grenade before the grenade was placed in the launcher. The safety pin could then be removed as the launcher cup kept the safety-lever in place. The operator inserted the ballistite cartridge into the rifle before setting the stock, angled on the ground to absorb the recoil of the weapon. When the cartridge was fired it pushed the grenade out of the cup releasing the lever. The cup-type launcher could launch the grenade about 200 yards (180 m). Lee–Enfield rifles were equipped with the cup launcher were sometimes modified with copper wire wrapped around the stock, to prevent the wood from splitting under the increased recoil. If necessary, both the rod and the gas check grenade could be thrown as a standard hand-grenade. The cup discharger was typically on issue to the British Home Guard rather than the regular British Army.
No. 5 Mk II Mills bomb
Cutaway view of a No. 5 Mills bomb
No. 23 Mk II Mills bomb
36M grenade dated 1940
Base of 36M grenade dated 1940
36M Mills bomb
Mills bomb No. 23 Mk II, with rod for launch by rifle
Drawing of the Mills No. 36 rifle grenade, with its gascheck disk for use with cup-launcher
Lee-Enfield cup-launcher in the 1916 Somme Battlefield Museum, France
A case of derived type with special base plug
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