The Mills bomb underwent numerous modifications. The No. 23 was a variant of the No. 5 with a rodded base plug which allowed it to be fired from a rifle. This concept further evolved with the No. 36, a variant with a detachable base plate to allow for use with a rifle discharger cup. The final variation of the Mills bomb was the No. 36M, which was specially designed and waterproofed with shellac for use in the hot climate of Mesopotamia in 1917. By 1918 the No. 5 and No. 23 were declared obsolete and the No. 36 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. Although the segmented body helps to create fragments when the grenade explodes, according to Mills' notes, the casing was grooved to make it easier to grip and not as an aid to fragmentation. The Mills was a defensive grenade: after throwing the user had to take cover immediately. A competent thrower could manage 30 meters with reasonable accuracy, but the grenade could throw lethal fragments further than this. It could be fitted with a flat base and fired with a blank cartridge from a rifle with a 'cup' attachment, giving it a range of around 150 m.
At first the grenade was fitted with a seven-second fuse to accommodate both hand and rifle launch, but during combat in the Battle of France in 1940, this delay proved too long - giving defenders time to escape the explosion, or even to throw the grenade back - and was reduced to four seconds.
The heavy, segmented bodies of 'pineapple' type grenades result in an unpredictable pattern of fragmentation. After the second world war Britain and the US 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 completely 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 early 1980s. That the Mills bomb remained in use for so many years says much about its effectiveness.