The Mk.2 Pineapple Grenade: A WWII Icon

The United States Mk.2 grenade, colloquially known as the “Pineapple” grenade due to its distinctively grooved exterior, is an iconic piece of military ordnance that served as the standard-issue hand grenade for the U.S. armed forces from World War I through the Vietnam War.

Its design, effectiveness in combat, and its presence in Hollywood movies have made it a legendary tool of war.

Development

Mk.1 Failure

Prior to the First World War, the concept of hand grenades was not new, but their use was limited and designs varied widely. The early stages of the First World War demonstrated the need for effective trench warfare weapons. The static nature of trench warfare, with opposing forces entrenched in close proximity, highlighted the utility of hand grenades for clearing enemy trenches and bunkers.

The U.S. entered the war with little experience in modern, large-scale warfare and lacked many of the advanced weapons that European powers had developed. The U.S. military’s initial hand grenade, the Mk.1, entered service in 1917, but proved to be inadequate for the demands of trench warfare.

The Mk.1 was heavily inspired by European designs.

It was a fragmentation grenade inspired by the British Mills Bomb and the French F1, but it suffered from reliability issues, and its design did not produce enough shrapnel to be effective in the close-quarters combat typical of trench warfare environments.

As fragmentation grenades, their primary means of inflicting damage against an enemy is with small pieces of shrapnel that are propelled from the grenade at high speed.

Need for the Mk.2

In response to the shortcomings of the Mk.1 and inspired by European designs, the U.S. Ordnance Department began developing a new hand grenade that would meet the needs of American forces. The result was the Mk.2 grenade, which featured a cast iron body designed to fragment upon detonation, creating a lethal radius of shrapnel.

This design was aimed at maximizing the grenade’s effectiveness in trench clearing and close combat situations. This was achieved with rectangular lumps that make up the grenade’s casing. Upon detonation, these lumps became shrapnel.

The Mk.2 grenade. The body is made from cast iron.

While later analysis showed that the grooves did not significantly affect shrapnel dispersion, they did provide a more secure grip and prevented the grenade from rolling on uneven surfaces, which were practical benefits in the muddy, chaotic environments of trench warfare.

The Mk.2 grenade’s design allowed for mass production, an essential factor given the enormous demand for munitions in observed during the First World War. Its relatively simple design, compared to more complex mechanisms in some contemporary grenades, facilitated rapid manufacturing and ease of use by soldiers with minimal training. This standardization was crucial for the U.S. military, which had to quickly equip a rapidly expanding force with effective weapons.

Despite its aim to replace the Mk.1, the Mk.2 only entered limited service during the First World War. Still, it soon proved to be a much more reliable design.

The grenade quickly became popular among US forces.

Mk.2 Grenade Design

The Mk.2 grenade’s body was made of cast iron, chosen for its ability to fragment upon detonation into lethal shrapnel. The exterior of the grenade featured a segmented pattern, initially intended to control the fragmentation and increase the grenade’s lethality.

Although later evaluations suggested that the grooves did not significantly influence the fragmentation pattern, they provided two unintended yet valuable benefits: enhanced grip and reduced roll. The textured surface allowed soldiers to hold and throw the grenade more securely, especially in wet or muddy conditions common in trench warfare. Additionally, the segmented design prevented the grenade from rolling away on uneven ground. This helped during transport, and keeping thrown grenade on target.

Diagram of the Mk.2.

It measured approximately 4.5 inches in height and 2.5 inches in diameter, with a weight of around 21 ounces (about 600 grams) when fully equipped with its explosive filler. This size and weight made the grenade manageable for most soldiers to throw, while still housing enough explosive material to be effective in combat situations.

Explosive Filler

The Mk.2 grenade was filled with a number of different explosive fillings over its service life. These ranged from Trojan nitrostarch to EC smokeless powder. Eventually these were replaced by more powerful and stable high-explosives like TNT. This transition significantly increased the grenade’s destructive potential. The choice of explosive material was a crucial aspect of the design, as it needed to provide a reliable and powerful blast to generate enough fragmentation effects.

Training variants were filled with black powder low explosives for a smaller blast.

Prior to WWII, high-explosive versions of the Mk.2 were painted yellow.

Fuse Mechanism

One of the most critical components of the Mk.2 grenade was its fuse mechanism, which underwent several iterations throughout its service life. The earliest versions were found to be unreliable and potentially hazardous. These were subsequently replaced by more sophisticated timed fuses, such as the M5, M6, and M204 series, which provided a more consistent and safer delay (typically 4 to 5 seconds) between the release of the lever (“spoon”) and detonation.

Fuses were inserted into a threaded hole in the top of the grenade. On later grenades this hole was also used to fill the grenade, compared to a plug in the bottom found on earlier types.

Blank training grenade.

Safety was a paramount consideration in the design of the Mk.2 grenade, with several features intended to prevent accidental detonation. The fuse was triggered by the removal of the “spoon”, a thin spring loaded lever that ran down the side of the grenade.

The spoon was held in place by a safety pin. Even with the pin removed, as long as the user holds down the spoon the grenade will not activate. To operate the grenade, the user must remove the safety pin by pulling on a ring.

Upon throwing the grenade, the spoon is ejected by a spring, activating a percussion primer in the fuse. This ignites an element that burns for 4-5 seconds, depending on the type, that will then ignite the main detonator for the explosives.

The grenades can often be seen hanging off the webbing of GIs.

One complaint about the Mk.2 was a quiet sound made by the fuse as the element burned, along with a small amount of smoke. This was largely why the Mk.2 would eventually be replaced.

Variants and Modifications

The Mk.2 came in a few different variants, mostly relating to the type of fuse and explosive filling inside. Externally, it remained virtually unchanged throughout its production.

Training versions of the grenade contained low explosives, allowing soldiers to practice with a device that simulated the weight and handling characteristics of the live grenade without the explosive hazard. These were painted black.

High explosive versions were painted yellow. During the Second World War, they were repainted to olive drab, with a small band of yellow left at the top of the cast body.

Tens of millions of Mk.2s were made.

Service

First World War

Introduced during the First World War, the Mk.2 grenade quickly became a valuable tool for American forces. The static nature of trench warfare necessitated weapons that could clear or disrupt enemy soldiers in fortified positions.

The ability to be thrown into the small, densely packed spaces of enemy trenches or bunkers made it fragmentation grenades great assets for infantry units. Its fragmentation design was particularly suited for the close-quarters combat of trench warfare, where the production of shrapnel could inflict casualties over a broad area.

Second World War

During the Second World War, the Mk.2 grenade saw much wider use compared to the First World War, serving across all theaters of operation, from the European to the Pacific fronts. The Mk.2 was utilized not only as an anti-personnel weapon but also for clearing machine gun nests, destroying enemy fortifications, and defending against attacking troops.

Seen hanging from the uniforms of GIs or packed into crates, the Mk.2 joined other equipment like the M1 Garand, Willys Jeep and P-51 Mustang as legendary symbols of the US military during the war.

Infantryman Fort Belvoir throwing a Mk.2 Grenade

One of the key strengths of the Mk.2 grenade was its versatility across the varied environments of the Second World War. In the dense forests and towns of Europe, the grenade was used to flush out enemy soldiers from cover, while in the Pacific theater, it was vital in combat against Japanese forces in jungles and island fortifications.

The grenade’s design, particularly its grooved exterior, facilitated this versatility by making it easy to handle and throw accurately, even in adverse weather conditions or when wearing gloves. Plus, it was small enough to be transported and carried in large numbers.

Millions of Mk.2 grenades were built from its entry into service during the First World War to the end of the Second World War. This meant that, despite being succeeded in the early 1950s, there were enough of them to see use in both the Korean and Vietnam Wars.

A GI sprays grenades.

Training and Handling

The effectiveness of the Mk.2 grenade in operational settings was heavily dependent on the training and proficiency of the soldiers using it. Training emphasized not only the mechanics of arming and throwing the grenade but also the tactics of using cover, timing throws for maximum effect, and coordinating grenade use with other infantry weapons. Soldiers were trained to throw grenades from a variety of positions, including standing, kneeling, and prone, to adapt to the realities of the battlefield.

Operational use of the Mk.2 grenade also highlighted the importance of safety and reliability. The development of safer fusing mechanisms and the addition of safety features, such as the safety clip, reduced the risk of accidental detonations. These improvements were critical in ensuring that the Mk.2 could be used effectively under the stress of combat conditions, where quick decision-making and reliability were paramount.

Pile of Mk.2 grenades.

By providing individual soldiers with the ability to deliver explosive force at a distance, the Mk.2 enhanced the lethality and versatility of infantry units. Its role in clearing enemy positions, disrupting troop movements, and inflicting casualties played a part in the tactical outcomes of numerous engagements.

Replacement

The Mk.2 was finally replaced in US service by the M26 grenade. Following the fruit-themed nicknames, the M26 known as the “lemon grenade” for its oval shape. The M26 offered improved reliability, safety, and fragmentation control compared to the Mk.2 “Pineapple”.

Its design featured a smooth outer casing and an internal fragmentation coil that produced a uniform fragmentation pattern, enhancing its lethality. It also improved on the Mk.2 by having a completely silent fuse that did not emit any smoke, decreasing the chances of being detected by the enemy.

An M26, the Mk.2’s replacement. It entered service during the Korean War.

The M26 weighed 450 g, and was filled with 165 g of Composition B, an explosive that detonates at higher velocities than TNT. The M26 was a staple in U.S. military operations until it was succeeded by near the end of the Vietnam War.

Other Interesting Grenades

MK1 Illumination Grenade

The Mk1 Illumination grenade is a device that creates an intense bright light to be used for illumination or as a signal. It is 4.4 inches tall and weighs 270 g. Visually, it closely resembles the M26 fragmentation grenade.

Mk 1 Illumination grenade diagram.

Inside, the grenade contains an illuminating compound and a black powder fuse. When the grenade’s pin is pulled, the fuse is triggered and detonates the black powder after a 7 second delay. This destroys the grenade’s casing, revealing and igniting the illuminating compound, which burns with a brightness of 50-60,000 candlepower for 25 seconds.

Offensive Grenade Mk IIIA2

Initial versions of this grenade entered service in 1918. They were created as conussion grenades that could be used against enemy forces in small spaces (trenches, bunkers etc.), where maximum damage could be caused without risking friendly forces to fragmentation effects.

The Mk IIIA2 is a later version, with a cylindrical pressed-fiber body that contained 230 g of TNT.