The Mills bomb became the standard British and Commonwealth hand grenade of the First World War and set the pattern for grenade design for decades. Reliable, safe to carry, and simple to use under stress, it gave ordinary infantrymen a powerful way to clear trenches, dugouts, and enemy positions at close range — exactly the kind of fighting that defined the Western Front.

Design and operation

The Mills bomb was a compact, cast-iron grenade with a distinctive grooved exterior, often nicknamed the “pineapple.” At its core was a central striker mechanism held in place by a spring-loaded hand lever and secured with a safety pin.

To use the grenade, the thrower removed the pin, kept pressure on the lever, and threw it. Once released, the lever flew off, allowing the striker to ignite the fuse. After several seconds, the main charge detonated.

Despite popular belief, the external grooves were not designed to control fragmentation. According to notes by William Mills, the grooves were intended solely to improve grip, especially in wet or muddy conditions. Testing has shown that the grenade casing does not reliably break apart along the segmented lines. Fragmentation was irregular, as with most cast-iron grenades of the period.

Variants and evolution

No. 5 The standard hand-thrown version used widely during the First World War.

No. 23 A modified No. 5 fitted with a rodded base plug, allowing it to be fired from a rifle. This extended the grenade’s range but added handling complexity.

No. 36 An improved design featuring a detachable base plate for use with a rifle discharger cup. This replaced the earlier rod system and made rifle launching safer and more practical while retaining hand-thrown capability.

No. 36M A further development specially waterproofed with shellac for service in hot and humid climates. First introduced for operations in Mesopotamia in 1917, it proved durable and reliable and remained in production for many years.

Service life and obsolescence

By 1918, the No. 5 and No. 23 had been declared obsolete.

The Mills bomb’s practical design and adaptability ensured its long service life and lasting influence on later grenade development.

Technical specifications

• Body material: Cast iron
• External form: Grooved (“pineapple”) casing intended to improve grip, not to control fragmentation
• Ignition system: Central striker with spring-loaded hand lever and safety pin
• Fuse type: Pyrotechnic time-delay fuse
• Typical fuse delay: Approximately 4–5 seconds (varied slightly by production batch and fuse type)
• Explosive filling: Amatol or similar high explosive (varied by production period)
• Weight: Approximately 1.25 lb (about 570 g), depending on mark and filling
• This grenade is inert