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Why old grenades used to have deep grooves? Why newer ones are completely smooth?

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Posted July 12, 2018

If you think of a hand grenade, most likely you imagine an old-school pineapple-looking weapon. Classic grenades used to have deep grooves dividing the outer surface into squares, which resembled a pineapple. But newer grenades are completely smooth. Why grenades used to have these grooves and what changed?

Very first hand grenades actually didn’t have these grooves, because they were improvised on the field. Soldiers used to make them from various cans and gun powder, but in 1915 William Mills invented a “safe” hand grenade with a manual detonator. Its purpose was quite simple – you pull the detonator activating the bomb, throw it somewhere close to the enemy and it explodes sending sharp metal shards to all directions, hopefully neutralizing soldiers from the opposing side. However, contrary to popular belief, grenades were never made to be powerful. They were made to be light, which meant that there was no way to fit a lot of explosives inside of them.

Mk2 grenade had deep grooves, making it look like the outer surface of a pineapple. Image credit: J-L Dubois via Wikimedia

Grenades have to be light and small for soldiers to carry and use them effectively. This means that there isn’t that much space inside. You could fill it fully with explosives alone, but they wouldn’t do much damage in an open field. A better solution is to rely on shrapnel flying out to all directions like bullets. Instead of adding some nails or other sharp metal objects inside, Mills decided to use the body of the grenade for the shrapnel. That is why he designed these deep grooves that were supposed to be weak points in which grenade breaks during an explosion.

These large metal squares were meant to become unpredictable bullets. In 1918 American Mk2 grenade came along and it also had quite deep grooves, dividing the outer surface into squares. Other countries also used this design solution, but some decades later new grenades came out completely smooth. Why?

Well, because grooves didn’t work. Mills noticed that grenade breaks into pieces in random locations and not necessarily where grooves are cut. That was because cast iron is quite brittle. In fact, so brittle that a large part of grenade’s body became iron powder upon the explosion and couldn’t inflict any damage to the enemy. Only around a third of grenades body became dangerous shrapnel. However, despite this being noticed quite early, many grenades kept their pineapple design.

Modern grenades like M67 are completely smooth on the outside, but have grooves on the inside. Image credit: Wikimedia

These grooves improved grip, allowing soldiers to handle and throw these grenades much easier. However, newer M26, M33 and M67 grenades had a completely smooth surface. Manufacturing of hand grenades changed, they started making grenade bodies from formed metal with grooves on the inside where they actually helped fragmentation. And soldiers are throwing them just fine, because new M67 is around 200 grams lighter than the old Mk2.

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