The ridges on classic grenades like the World War II “pineapple” grenade were designed to help the casing break apart into fragments on detonation. That’s the textbook answer, and it’s partially true, but the full story is more interesting. Those exterior grooves didn’t actually control fragmentation as well as engineers hoped, and modern grenades have largely abandoned the design in favor of better solutions hidden inside.
The Original Idea Behind the Ridges
When engineers designed early fragmentation grenades, they faced a basic physics problem: a smooth metal shell, when blown apart by an explosive charge, breaks into unpredictable chunks. Some pieces are large and heavy, others are tiny and harmless, and the spread pattern is uneven. The solution seemed intuitive. Score the outside of the casing with deep grooves, creating a grid pattern, and the metal should fracture along those lines into uniform, lethal fragments.
The Mk 2 grenade, the iconic “pineapple” used by U.S. forces from 1918 through the Vietnam War, is the most recognizable example of this approach. Its cast iron body featured rows of raised rectangular segments separated by deep channels. The idea was that each of those roughly 40 segments would break off as an individual fragment, creating a predictable kill radius.
Why the Ridges Didn’t Work as Planned
Battlefield testing and engineering analysis revealed a problem. When an explosive charge detonates inside a metal shell, the forces involved are so extreme that the casing doesn’t fracture neatly along the grooves. The metal shatters somewhat randomly, with fracture lines running through the ridged segments just as often as along them. The grooves weakened the casing at certain points, which had some influence on fragmentation, but the fragments produced were far less uniform than designers intended.
The result was that the ridged exterior was more cosmetic than functional. It looked like it should work, and it did provide a modest degree of fragmentation control, but it fell well short of producing the reliable, evenly sized fragments that military engineers wanted.
Grip Was a Secondary Benefit
One practical advantage of the ridged design was that it gave soldiers a better grip. A textured surface is easier to hold than a smooth sphere, especially with wet, muddy, or gloved hands. Research conducted for the U.S. military on grenade throwing accuracy found that spherical grenades were significantly less accurate to throw than elongated or textured shapes. Oblong objects fit the contour of the hand better and are easier to control during the throwing motion. The three non-spherical shapes tested were essentially equal in accuracy, but the sphere performed notably worse.
So while grip wasn’t the primary reason for the ridges, it was a real benefit that kept the design around longer than the fragmentation logic alone might have justified.
How Modern Grenades Handle Fragmentation
The M67 grenade, which replaced the Mk 2 in 1968 and remains the standard U.S. fragmentation grenade, has a smooth, round steel body with no exterior ridges at all. Instead of relying on the outer casing to fragment predictably, modern grenades use engineered solutions on the inside.
One common approach is a notched wire coiled in a spiral inside the grenade body. The wire is essentially a long steel bar with dimensions matching the desired fragment size, scored at regular intervals along its length. When the explosive detonates, the wire breaks at those notches into small, uniform pieces that fly outward as shrapnel. Other designs use notched rings stacked inside the casing, each one pre-scored to break into fragments of a specific shape and weight.
The German DM-41 grenade, for example, uses a grooved steel sleeve wrapped around the explosive charge. That sleeve contains over 1,000 grooves across 31 wraps of steel, producing roughly 2,000 to 2,100 individual fragments per grenade. The base plug alone contains 33 to 36 additional round steel fragments. This internal engineering gives far more precise control over fragment count, size, and distribution than exterior ridges ever could.
The principle behind all these methods is the same: score or notch the metal before detonation so it breaks along predetermined lines when the explosive force hits. Doing this on the inside, where the geometry can be tightly controlled and protected from damage, works far better than doing it on the outside.
Why the Pineapple Shape Stuck Around So Long
If the ridges didn’t fragment reliably, why did the military use them for decades? Partly because they worked well enough. The Mk 2 was lethal and battle-proven through two world wars and Korea. The ridges did provide some fragmentation benefit, even if it was inconsistent, and the grip advantage was real. Manufacturing technology also played a role. Cast iron with exterior grooves was simple and cheap to produce in enormous quantities, which mattered when the U.S. was manufacturing grenades by the millions.
By the late 1960s, materials science and munitions engineering had advanced enough to make internal fragmentation systems practical for mass production. The M67’s smooth exterior and internal fragmentation design represented a genuine leap in lethality and consistency. Even so, the Army has acknowledged the M67 has its own limitations. Soldier feedback from Iraq and Afghanistan identified problems with fragmentation performance in dense vegetation and wooded terrain, and the military has been working on next-generation designs that improve both safety and lethality.
The Short Answer
Grenades have ridges because early engineers believed exterior grooves would control how the casing shattered into fragments. The grooves helped somewhat but didn’t produce the reliable fragmentation pattern intended. They also improved grip for throwing. Modern grenades have moved to smooth exteriors with precision-engineered internal fragmentation systems, making the classic pineapple shape a relic of an era when the best available solution happened to look like it should work better than it actually did.