US Mk2 Fragmentation Grenade

Overview

The Mk 2 Fragmentation Grenade, universally known as the “pineapple grenade,” is one of the most iconic weapons of the 20th century. This hand grenade served as the standard US fragmentation grenade from 1918 through the 1960s, seeing combat in both World Wars, Korea, and Vietnam. Its distinctive serrated cast-iron body, designed to enhance fragmentation, made it instantly recognizable and established the visual archetype of what many people picture when they think of a hand grenade. Despite being obsolete for decades, the Mk 2 remains significant in ordnance recognition due to its continued presence in stockpiles worldwide and its cultural prominence in military history.

Country/Bloc of Origin

Country: United States of America
Development Period: 1917-1918 (World War I era)
International Variants: Licensed or copied by numerous nations including China, Taiwan, and various South American countries
Bloc Adoption: Widely distributed to Allied and pro-Western forces throughout the Cold War

Ordnance Class

Type: Hand Grenade
Primary Role: Anti-personnel fragmentation
Delivery Method: Hand-thrown
Classification: Defensive fragmentation grenade (thrower requires cover due to large fragmentation radius)

Ordnance Family/Nomenclature

Official Designations:

Mk 2 (Mark 2) – Primary US designation
Mk II (alternative spelling in some documentation)
Hand Grenade, Fragmentation, Mk 2

NATO Stock Numbers:

Various NSNs depending on fuze configuration
1330-00-773-8529 (with M6 series fuze – common variant)

Related Variants:

Mk 1 (predecessor with inferior fragmentation)
Mk 2A1 (improved fragmentation variant)
T13 Beano Grenade (impact-fuzed variant, experimental)
M16A1/M16A2 (rifle grenade variants using Mk 2 body)

Common Names and Nicknames:

“Pineapple” or “Pineapple Grenade” (most common)
“Fragmentation Grenade”
“Mk 2 Frag”
“Lemon” (British term, less common)

International Designations:

Type 23 (Chinese copy)
Various local designations in countries that manufactured copies

Hazards

The Mk 2 presents significant fragmentation and blast hazards:

Primary Hazard Types:

Fragmentation: Cast iron body produces approximately 1,000 fragments traveling at lethal velocities
Blast Overpressure: TNT or Composition B creates significant blast effect
Incendiary Effect: Minimal but present; can ignite flammable materials

Fragmentation Characteristics:

Effective Casualty Radius: 10 meters (32 feet)
Fragmentation Dispersal: Up to 230 meters, with lethal fragments to 50+ meters
Fragment Velocity: Initial velocities up to 1,200 meters/second
Fragment Size: Highly variable; from sub-gram particles to multi-gram chunks

Sensitivity Factors:

Impact Sensitivity: Low in normal state; fuze is percussion-activated
Shock Sensitivity: Explosive fill relatively stable but can detonate if involved in fire or subjected to severe shock
Aging Degradation: TNT sweats out in very old specimens; degraded explosives may be more sensitive

Unexploded Ordnance (UXO) Considerations:

Common UXO find at former US military training ranges
Fuze mechanisms can deteriorate unpredictably with age
May be found with corroded bodies but still containing live explosive
Some specimens may have damaged fuzes that are unstable
Practice variants (blue body) exist and should be positively identified before assuming inert status

Safety Distances:

Minimum Safe Distance: 40 meters in open terrain (defensive grenade requires cover for thrower)
Training Safe Distance: 100+ meters for observation of live throws
UXO Safety Cordon: Minimum 300 meters until ordnance disposal team arrives

Key Identification Features

Physical Dimensions:

Length: 4.5 inches (114 mm) including fuze
Body Diameter: 2.3 inches (58 mm)
Weight: 21 ounces (595 grams) fully assembled
Body Weight: Approximately 5 ounces without fuze and filler

Shape and Profile:

Distinctive oval “pineapple” shape with vertical and horizontal serrations
Fuze assembly protrudes from top with threaded connection
Flat or rounded bottom (depending on manufacturing variant)
Pronounced external segmentation pattern

Color Schemes and Markings:

Combat Models: Olive drab (OD) green or gray-green
Practice Models: Light blue with black markings
High Explosive (HE) Identifier: Yellow band around body (on some variants)
Markings: Typically include lot numbers, manufacturer codes, date stamps
Stenciling: Black paint used for text markings

Distinctive External Features:

Serrated Body: 40-50 knobs or squares arranged in offset rows
Threaded Fuze Well: Central threaded opening at top
Segmented Pattern: Creates the characteristic “pineapple” texture
Casting Marks: Seams and manufacturing imperfections visible on cast body
Eyebolt/Hook: Some variants have mounting point at bottom

Material Composition:

Body: Cast iron (typical) or cast steel (some variants)
Fuze: Brass, steel, and aluminum components
Safety Components: Steel wire and pull ring

Visual Recognition at Distance:

Distinctive segmented oval profile
Slightly larger and heavier than modern smooth-bodied grenades
Pronounced surface texture visible even when partially obscured

Fuzing Mechanisms

The Mk 2 was employed with several different fuze types throughout its service life:

Primary Fuze Types:

1. M6 Series Fuze (Most Common):

Pyrotechnic delay fuze
4-5 second delay from lever release to detonation
Percussion primer initiated

2. M5 Series Fuze:

Earlier pyrotechnic delay design
Similar 4-5 second delay
Less reliable than M6 in adverse conditions

3. M10 Series Fuze:

Improved weatherproofing
More consistent delay timing
Used in later production

Arming Sequence:

Safety clip removed (if present)
Pull ring withdrawn, removing safety pin
Safety lever (spoon) held in place by thrower’s grip
Upon release of lever, striker spring drives striker against primer
Primer ignites delay element
4-5 second delay train burns
Delay train ignites detonator
Detonator initiates main explosive charge

Safety Mechanisms:

Safety Pin: Prevents lever from rotating off body
Safety Clip: Additional safety on later fuzes; must be straightened/removed first
Positive Striker Block: Mechanical block prevents striker from contacting primer until lever is released
Delay Element: Provides time for thrower to seek cover

Triggering Method:

Impact percussion system (striker hits primer)
Time delay only (no impact or proximity features)
Irreversible once spoon is released
No anti-handling or booby-trap features in standard configuration

Reliability Issues:

Older fuzes may have degraded delay elements
Moisture penetration can cause fuze failure
Some batches known to have unreliable timing
Cold weather can affect delay burn rate

History of Development and Use

Development Origins:

The Mk 2 was developed in 1917-1918 during World War I as the United States sought to improve upon the Mk 1 grenade, which had poor fragmentation characteristics. The design team at the Picatinny Arsenal drew inspiration from the French F1 grenade and British Mills bomb, both of which used serrated bodies to enhance fragmentation.

Design Philosophy:

Create predictable, effective fragmentation through body segmentation
Standardize on proven pyrotechnic delay fuze
Achieve maximum lethality within hand-throwing range
Enable mass production through casting technology

World War I Service (1918): The Mk 2 entered service in the final months of World War I but saw limited combat use before the armistice. However, it became the standard US fragmentation grenade and remained in production and stockpiling through the interwar period.

World War II (1941-1945): The Mk 2’s finest hour came during World War II, where it saw extensive combat across all theaters:

Pacific Theater: Essential for close-quarters jungle fighting and island assaults; clearing bunkers and caves on islands like Iwo Jima and Okinawa
European Theater: Used in urban combat, trench clearing, and defensive operations from Normandy to Germany
Production Numbers: Millions produced; exact figures vary but estimates exceed 75 million grenades
Tactical Role: Primary hand grenade for US forces; carried by virtually all infantry personnel
Combat Effectiveness: Proven highly effective for bunker clearing, room clearing, and point defense

Korean War (1950-1953): The Mk 2 continued as the standard US grenade during the Korean War:

Effective in mountainous terrain and winter conditions
Used extensively in tunnel warfare and bunker assaults
Some fuze reliability issues noted in extreme cold
Supplemented by rifle grenade variants for extended range

Vietnam War (1955-1975): The Mk 2 saw the beginning of its obsolescence during Vietnam:

Gradually replaced by the M26 and M33 series grenades
Still in widespread use, particularly in early war years
Complaints about weight and throwing difficulty in jungle conditions
Some units continued using Mk 2 stocks into the late 1960s
Captured Mk 2s used by North Vietnamese and Viet Cong forces

Phaseout and Replacement:

1960s: Formal replacement by M26 series began
1970s: Mk 2 removed from frontline service in US forces
Legacy Stocks: Remained in US strategic reserves until the 1990s
International Use: Many allied nations continued using Mk 2s for decades after US retirement

Global Distribution: The Mk 2 was distributed globally through:

Lend-Lease programs in WWII
Post-war military aid packages
NATO standardization (until superseded)
Foreign military sales programs
Captured stocks redistributed by various forces

Current Status:

Obsolete: No longer in US military service
Training Aid: Used in ordnance recognition and museum displays
UXO Hazard: Found at former training sites and battle areas
International Service: Some copies still in service with militaries that received them decades ago
Collectors’ Item: Inert examples common in military collections

Impact on Warfare: The Mk 2 established the template for fragmentation grenades and became synonymous with hand grenade warfare in popular culture. Its effectiveness drove tactical innovations in close-quarters combat, particularly in WWII urban and jungle fighting. The grenade’s design influenced numerous foreign copies and inspired the development of modern fragmentation grenades worldwide.

Technical Specifications

Explosive Fill:

Fill Type: TNT (Trinitrotoluene) or Composition B
Fill Weight: 2 ounces (57 grams) TNT typical
Alternative Fills: Some wartime production used EC (Explosive Composition) blends
Detonator: Tetryl or lead azide booster pellet in fuze

Fragmentation Performance:

Fragment Count: Approximately 1,000 fragments from body segmentation
Fragment Pattern: Irregular; influenced by casting defects and body thickness variations
Primary Fragment Weight: 0.2-5 grams per fragment (highly variable)
Lethal Range: 50+ meters for fragments
Effective Casualty Radius: 10 meters

Blast Characteristics:

Blast Pressure: Moderate; sufficient to cause injury at close range
Blast Radius: Lethal overpressure within 2-3 meters
Combined Effects: Fragmentation is primary kill mechanism; blast is secondary

Operational Specifications:

Maximum Throwing Range: 35-40 meters by average soldier
Effective Range: 25-30 meters (to ensure grenade detonates beyond thrower’s position)
Fuze Delay: 4-5 seconds from lever release
Body Fragmentation: Irregular pattern; not controlled fragmentation like modern grenades

Environmental Performance:

Operating Temperature: -40°F to +140°F (-40°C to +60°C) nominally
Extreme Cold Issues: Fuze delay can lengthen; explosive fill becomes more brittle
Humidity Effects: Moisture can penetrate fuze assembly; corrosion of internal components
Shelf Life: Indefinite for body; fuzes have finite reliability period (10-20 years typical)

Physical Robustness:

Drop Resistance: Can withstand moderate drops with safety pin installed
Rough Handling: Cast iron body resistant to deformation but can crack
Storage Durability: Extremely durable if kept dry; iron body corrodes in wet conditions
Fuze Fragility: Fuze components more delicate than body; subject to shock damage

Frequently Asked Questions

Q: Why is the Mk 2 called the “pineapple grenade”?

A: The Mk 2 earned its “pineapple” nickname from its distinctive segmented cast-iron body, which resembles the knobby exterior texture of a pineapple fruit. This segmentation, consisting of approximately 40-50 raised squares or knobs arranged in offset rows, was designed to enhance fragmentation by creating predetermined weak points in the cast iron body. When the grenade detonates, the explosive force causes the body to break apart along these segmented lines, theoretically producing more uniform and effective fragmentation. The name became so universally recognized that “pineapple grenade” is often used colloquially to refer to any old-style serrated grenade, even though the Mk 2 specifically refers to the US version.

Q: Was the segmented “pineapple” design actually effective at improving fragmentation?

A: The effectiveness of the serrations is actually a matter of historical debate among ordnance experts. While the design intent was to create controlled fragmentation along the grooves, testing revealed that fragmentation patterns were largely random and influenced more by casting flaws, iron grain structure, and variable body thickness than by the external serrations. The serrations provided psychological confidence to users and may have created marginal improvements in fragment count, but modern smooth-bodied grenades with internal fragmentation coils or notched wire achieve far more predictable and effective fragmentation. The Mk 2’s effectiveness came more from its substantial explosive fill and heavy cast-iron body mass than from the specific segmentation pattern. Nonetheless, the design was combat-proven over multiple wars and achieved its fundamental purpose of producing lethal fragmentation.

Q: Why did the US military eventually replace the Mk 2 with the M26 series?

A: The Mk 2 was replaced for several important reasons. First, it was significantly heavier (21 ounces) than necessary, making it tiring to carry in quantity and difficult for smaller soldiers to throw effectively—a particular issue in Vietnam where troops often carried 30+ pounds of ammunition and equipment in jungle heat. Second, the unpredictable fragmentation pattern meant some detonations produced fewer effective fragments than desired. Third, the cast iron body was expensive and time-consuming to manufacture compared to stamped steel designs. Fourth, the M26 introduced a more reliable fragmentation system using an internal notched wire coil that produced more consistent, lethal fragments. Finally, modern warfare emphasized lighter individual loads and the M26’s reduced weight (14 ounces versus 21) was significant when multiplied across a soldier’s combat load. The replacement reflected lessons learned about balancing lethality, manufacturability, and practical combat ergonomics.

Q: How dangerous is it to encounter an old Mk 2 grenade, and what should someone do?

A: Old Mk 2 grenades are extremely dangerous and should never be handled by untrained personnel. Even grenades that appear corroded or damaged may contain live explosive fill and functional fuze components. The explosive filling (TNT or Composition B) remains potent for decades, and aged explosives can actually become more sensitive due to chemical degradation. Fuze mechanisms can corrode in ways that make them unpredictable—a fuze that appears to be “frozen” might actually be extremely sensitive to shock or vibration. If you encounter a suspected Mk 2 grenade, maintain a safe distance of at least 300 meters, mark the location if possible, prevent others from approaching, and immediately contact local law enforcement or military authorities who will dispatch an Explosive Ordnance Disposal (EOD) team. Never attempt to move, clean, disassemble, or photograph it closely. Training ranges, battlefields, and even old farms in areas of WWII combat can still yield these devices.

Q: Can you legally own a Mk 2 grenade, and how do collectors obtain inert examples?

A: In the United States, ownership of a functional Mk 2 grenade is strictly regulated under the National Firearms Act (NFA) and requires registration as a “destructive device” with ATF, extensive background checks, and a $200 tax stamp—and even then, transfers of explosive ordnance to civilians are rarely approved. However, completely inert or demilitarized Mk 2 grenades are legal to own in most jurisdictions. Inert examples must have the explosive fill removed, fuze well drilled out or plugged, and often have a hole drilled through the body to demonstrate that they cannot hold explosives. Reputable military surplus dealers sell verified inert grenades, often accompanied by documentation of demilitarization. Collectors should be extremely cautious when purchasing—some grenades sold as “inert” have been improperly deactivated or are actually live. Always purchase from established dealers who provide demilitarization certification, and never attempt to verify inertness yourself. Laws vary by state and country, so collectors should verify local regulations before acquiring even inert ordnance.

Q: How does the Mk 2 compare to the German “potato masher” stick grenade used in WWII?

A: The Mk 2 and the German Model 24 Stielhandgranate (“stick grenade” or “potato masher”) represented fundamentally different design philosophies. The Mk 2 was a defensive fragmentation grenade—it produced extensive fragmentation from its heavy cast-iron body, requiring the thrower to take cover due to the large fragment dispersal radius. The stick grenade was an offensive blast grenade with minimal fragmentation from its thin sheet-metal head, designed so troops could throw it and immediately advance without needing cover. The stick grenade’s long wooden handle provided superior leverage for throwing distance (40-50 meters versus 25-35 meters for the Mk 2), but it was bulky to carry. The Mk 2 was more lethal against exposed troops due to fragmentation but heavier and harder to throw. In practice, American troops who captured stick grenades appreciated their throwing distance, while German troops who captured Mk 2s valued their lethality. Each design excelled in its intended role—the Mk 2 for defensive positions and close-quarters combat, the stick grenade for offensive assault and longer-range engagement.

Q: Were there any variants or modifications to the Mk 2 during its long service life?

A: Yes, the Mk 2 saw several variants and modifications over its 50+ year service life. The Mk 2A1 variant featured improved internal fragmentation design with slightly different body casting. Fuze types evolved considerably—the original M5 fuze was supplemented and eventually replaced by the more reliable M6 series, and later by weatherproofed M10 fuzes. The M16A1 and M16A2 rifle grenade variants used the Mk 2 body with modified fuzing and a launching adapter, allowing it to be projected from the M1 Garand or M1 Carbine. Various manufacturers produced Mk 2s with subtle differences in body casting, threading, and dimensions, though all adhered to the same basic specifications. During WWII, production shortcuts were sometimes taken—some grenades received different explosive fills due to material shortages, and manufacturing quality varied between contractors. Practice versions (blue body, inert or reduced charge) were produced for training. Post-war, some grenades were modified with different fuzes as older stocks were refurbished. Foreign copies introduced their own variations, with Chinese Type 23 grenades being notably different in quality and dimensions despite similar appearance.

Q: Why was the Mk 2 considered a “defensive” grenade versus an “offensive” grenade?

A: The Mk 2’s classification as a defensive grenade stems from its extensive fragmentation radius, which exceeds the distance an average soldier can throw it. With lethal fragments dispersing up to 50 meters or more and the effective casualty radius around 10 meters, a thrower in open terrain risks self-injury from their own grenade if they throw from an exposed position and don’t immediately take cover. This makes the Mk 2 ideal for defensive situations where troops throw from prepared positions, foxholes, or behind cover—they can lob the grenade at attacking enemies while protected from the resulting fragmentation. In contrast, an offensive grenade (like the later M34 white phosphorus or concussion grenades) produces primarily blast effects with minimal fragmentation, allowing attacking troops to throw the grenade and immediately advance without waiting for fragments to clear. The defensive/offensive classification relates to tactical employment, not physical orientation. The Mk 2’s design prioritized maximum lethality against exposed enemy troops rather than safe employment by troops in the open, making it perfectly suited for defensive fighting but less ideal for rapid assault operations.

This lesson is intended for educational and training purposes. All ordnance should be considered dangerous until proven safe by qualified personnel. Unexploded ordnance should never be handled by untrained individuals—report findings to military or law enforcement authorities.