The Mills Bomb

The No.5 grenade was designed by Sunderland-born engineer William Mills and brought into service with the British Army in May 1915. More commonly known as the Mills Bomb, the No.5 was a percussion cap fragmentation grenade that was made from cast iron and most commonly filled with the explosives, ammonal or amatol. It proved a robust and effective weapon on the battlefield and, with subsequent variants and marks, became the standard hand grenade for the remainder of the First World War.

First produced at the Mills Munition Factory in Birmingham, the grenade was loosley-based on a design by Leon Roland, an officer in the Belgian army who was captured in November 1914, and his compatriot Albert Dewandre. Mills improved their early prototype by adding his own unique elements and produced a design so successful in field trials that production of the grenade, designated the No.5 Mark I, began in early 1915.

Featuring a distinctive cast iron segmented “pineapple” body, the No.5 contained a central firing striker that was held by an external lever and secured with a safety-pin. It was usually filled with ammonal, an explosive mixture consisting of TNT, ammonium nitrate and aluminium powder, and had a time-delay fuse of 4.5 seconds. To fire the No.5, it was held with the striker level firmly against its body while the safety pin was removed by its circular ring. The grenade was then thrown, releasing the pressure from the striker lever to initiate the time-delay fuse.

Two variants of the No.5 were produced during the war that enabled it to be fired from a rifle. The No.23 allowed an 8 inch firing rod to be screwed into its base plug, while its successor, the No.36, featured a gas-check disc on its base that was designed to be used with a discharger cup attached to the muzzle of the Short Magazine Lee-Enfield. A further variant of the No.36 was then produced which was sealed with shellac to prevent corrosion of the grenade and its detonator fuse. Initially designed for use in Mesopotamia, the No.36M was introduced in 1917 and would remain in service with the British Army for the next six decades. The variants were commonly filled with amatol before being the explosive was replaced at a later date by baratol.