Some 33-50 Million were produced by around 100 contractors during WWI .. initially made with a seven second fuse .. this proved to be too slow - permitting grenades to be thrown back - so was changed to a four seconds only fuse. According to Mill's notes - the "pineapple" grooving is NOT for fragmentation but for security of grip - and in practice it has been demonstrated that the casing randomly fragments into irregular shrapnel. The British Home Guard were instructed that while these grenades might be accurately thrown 30 yards .. the danger area was 100 yards - thus requiring the user to take cover.
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. According to Mills's notes, the casing was grooved to make it easier to grip and not as an aid to fragmentation, and in practice it has been demonstrated that it does not shatter along the segmented lines.
The heavy segmented bodies of "pineapple" type grenades result in an unpredictable pattern of fragmentation. After the Second World War Britain adopted grenades that contained segmented coiled wire in smooth metal casings.
The 'Mills' was soon adapted as a rifle grenade to extend it's range - first by the fitting of a rod that went down the Lee-Enfield rifle barrel - the pin was pulled - then the rifle fired with a "blank" cartridge propellant.
Mills Rifle Grenade No.23 Mk.II
This in practice was found to be an unsatisfactory & destructive (!!) system .. leading to development of the much better "CUP Grenade" and launcher.
Mills Cup Grenade No.36. With Gas Check Disc.
The Cup-Grenade launcher rifle was first loaded with the special projector 'blank' cartridge - the 'position' was adopted - then the pin pulled while - the insertion into the cup retained the grenade's hand lever on-safe until it was launched.
Mills Rifle Cup Grenade Firing Position
- That's Better!
.. The various versions of the Mills Grenade adapted for launching by rifle could still be used as Hand Grenades. “it is essential the lever be held securely against the grenade, otherwise the collar that holds the striker may release and ignite the fuse.”
In 1917 the British copied the German "Egg" grenade producing the No.34 Mks. I, II, III, & Mk.IV "Testicle" grenade. This was smaller & lighter than the 'Mills' and was better suited to combat from the trenches.
No.34 Cast Iron Testicle (Egg) Grenade.
There is a much wider variety of grenades & bombs than those I have researched .. including phosphorus types etc. - my interest is really in ballistics rather than bombs. - For a very good (82 page) read about THE HAND GRENADE by Gordon Rottman - follow this link to a nicely illustrated pdf. LINK:
TFB reports that US Army has dropped competency at throwing grenades as a basic training requirement due to "lack of skills & discipline in trainees".