"Mills bomb" is the popular name for a series of British hand grenades which were designed by William Mills. 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. The Roland grenade was a segmented sphere with multiple internal chambers for the fuze and filled with cast iron segments, which made it a complex and expensive design. In January 1915, the Mills Factory made four Roland grenades for evaluation by the War Office; during testing the grenades failed to detonate properly and was promptly rejected as unreliable and unsafe.
Mills continued working on the Roland design and by early February 1915, he made some technical drawings. The first proposed design had a cylindrical shape with external serrations instead of the spherical or oval shapes used by the Roland and the production Mills grenades respectively; the first prototypes were tested on 20 February 1915, and with a few modifications the Mills Bomb was soon approved mass production, with the British government placing orders for 150,000 grenades in April 1915.
Col. Arthur Morrow, a New Zealand Wars officer, also believed aspects of his patent were incorporated into the Mills Bomb.
Besides slow initial production rates, which forced the British to standardize improvised designs such as the Jam tin grenade and the Hairbrush grenade for mass production as a stopgap measure, the Mills bomb No. 5 also suffered from some design flaws, including premature detonations and a poorly designed safety pin, which could be accidentally dislodged with a minimal amount of movement, leading to modifications to the design itself and training procedures; The No. 5 was later modified as the No. 23, which could be either thrown by hand or launched as a rifle grenade.
Due the No. 5 initial flaws, the British worked on the No. 54 grenade as a replacement from 1916 until 1935, when the Army decided to retain the improved and combat-tested Mills bomb No. 36M in service.
The Mills was long and in diameter filled with just over of explosive filler: Alumatol, Ammonal, Baratol, Bellite, Cilferite, Sabulite, or Trotyl, while the grenade body was coated inside and out with shellac to prevent rusting and chemical reactions with some fillers. Combat experience during WWI showed that the Trotyl and Amatol fillings used in France weren't suitable for the heat of Mesopotamia, which led to the development of the Baratol-filled No. 23M grenade. All Cilferite-filled grenades were declared unsafe to use and disposed of in 1932 and from 1936, Trotyl was no longer used as filler.
According to Mills's notes, the casing was grooved to make it easier for soldiers to grip the grenade with muddy hands, not as an aid to fragmentation; and experiments conducted during World War II conclusively proved that external serrations have no effect whatsoever in controlling fragmentation.
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 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 with a danger area of about .
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 .
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 than 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 .
LeeâÂÂEnfield rifles 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.
The Mills bomb was adopted by the British Army as its standard hand grenade in 1915 as the No. 5. According to Ian V. Hogg, the No. 5 and No. 23 were withdrawn from service in 1918, while the No. 36 was declared obsolete in 1936. All Cilferite-filled grenades were declared as unsafe and disposed of by the end of 1932.
Ian V. Hogg also mentions that as early as 1916, the British were trying to develop a replacement for the Mills bomb, the No. 54 until development was halted in 1935 and the Army retained the combat-proven No. 36M. It was not until the late 1960s that a replacement was approved, the L2A1 grenade. The 36M remained in sporadic production until 1972.
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.
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).