British Mills Bomb No 36M Mk 1 INERT

Pitting 1940

British Mills Bomb No 36M Mk 1 INERT

British "Mills Bomb" No.5, No.23, No.36, WWI - WWII

Overview
Without a doubt the most successful hand grenade of the 20th Century, having the longest continuous service life of any other type.

While it may not be an ideal design for most efficient function (offset detonator, causing asymmetrical fragmentation) it was simple, rugged and effective.

The Mills Bomb is arguably the iconic example of a 20th century Hand Grenade.

The history behind the "Mills Bomb" is rather involved, starting with the first concept patented by the Belgian Army in 1912 (The 'Roland Grenade'), evolving to the Mills Patent 'Grenade, Hand, No.5' in 1915. Production commenced swiftly with over 75 million Mills grenades manufactured between April 1915 and late 1918.

The No.36 Mk.I version was introduced at the end of WWI and remained the standard British fragmentation grenade until 1972. The Mills Bomb was in service for over 55 years. It has been copied by at least a half dozen other countries and was still in active foreign service (Pakistan) through the early 1980's.

The term "Mills Bomb" was first coined early in WWI, as grenadiers were referred to as "bombers". The term was confusing and soon "grenade" was officially applied, but the old term continues to be used interchangeably.

Function
The Mill Bomb mechanism is functionally the same for the No.5, No.23 or No.36. The "centrepiece" holds the fuze mechanism and detonator while providing a sealed chamber for the explosive, which is poured in through a fill plug on the top side. The fill plug and centrepiece threads are usually coated with a waterproof material to provide a weather tight seal. (This also creates difficulties when trying to inert a live grenade, since it is common for those parts to become tightly bonded, proving difficult or impossible, to unscrew.)

The fuze consists of a percussion cap, delay and detonator tube, crimped and formed into a 'U' shaped unit. Prior to use it was inserted into the base of the centrepiece and closed tight with a base plug.

The grenade is designed with a strong coil spring which drives the striker. The striker is held in a cocked position by the safety lever. A safety pin locks the lever in place.

Body Design
The grenade number was changed to indicate overall functionality.
No.5 - For hand use only.
No.23 - Rod grenade capability, by changing the base plug, and developing an appropriate launching cradle (cup).
No.36 - Modified for cup launching, another base plug change, along with the creation of a suitable cup discharger. Internal and external mechanical components underwent many modifications. Some were associated with a particular Number or Mark, others changed independently during the course of production.

Early grenades, No.5 and No.23 Mk.I & II (left) are easily distinguished from the later No.23 Mk.III and No.36 (right) by examining the top features of the grenade body. Note the striker head style as well.

The No.5, 23 and 36 were all developed during WWI. Only the No.36 saw service in WWII.