US Mk3A2 Offensive Hand Grenade

1. Overview

The Mk3A2 Offensive Hand Grenade is a cylindrical concussion (blast) grenade designed by the United States to produce casualties during close combat while minimizing danger to friendly personnel from fragmentation. Commonly referred to as the “concussion grenade” or “demo grenade,” the Mk3A2 relies on blast overpressure rather than fragmentation as its primary casualty-producing mechanism. It is particularly effective when employed in enclosed spaces such as bunkers, buildings, tunnels, and fortified positions, where the shock waves reverberate off walls to devastating effect.

⚠ SAFETY WARNING: All ordnance should be considered dangerous until rendered safe by qualified EOD personnel. Unexploded ordnance should never be handled by untrained individuals. Report all suspected ordnance to military or law enforcement authorities. This information is for educational and identification training purposes only.

2. Country/Bloc of Origin

  • Country: United States of America
  • Developed By: U.S. Army Ordnance Department
  • Period of Development: The Mk3 series originated in March 1918 (World War I). The Mk3A2 is the final improved variant of this lineage.
  • Service Period: The Mk3A2 served from World War II through Vietnam and into the post-Vietnam era. It was formally withdrawn from the U.S. inventory in 1975 due to the asbestos content in its exterior coating and body materials.

3. Ordnance Class

  • Type: Hand grenade
  • Primary Role: Offensive, concussion/blast — designed for use in close combat and enclosed spaces
  • Delivery Method: Hand-thrown
  • Secondary Roles: Demolition, breaching, underwater denial (fishing/swimmer deterrent in field-expedient use)

4. Ordnance Family / Nomenclature

  • Official Designation: Grenade, Hand, Offensive, Mk3A2
  • Markings: “GRENADE HAND — OFFENSIVE MK3A2 — TNT” in yellow lettering on the body
  • Common Names/Nicknames: Concussion grenade, Demo grenade
  • Related Variants:
    • Mk3 — Original WWI-era variant; cardboard body with crimped metal top and bottom; used the Mk.II fuze
    • Mk3A1 — Improved variant with water-resistant laminated-paper body and die-cast metal caps; used M6-series fuzes; 7 oz TNT fill
    • Mk3A2 — Final variant; waterproof asphalt-impregnated fiberboard body; 8 oz TNT fill; used M206-series fuzes
  • Modern Replacement: The M111 Offensive Hand Grenade, adopted by the U.S. Army in 2021, replaces the Mk3A2 for blast overpressure and concussion effects. The Enhanced Tactical Multi-Purpose (ET-MP) grenade is also being developed to combine fragmentation and concussive effects.

5. Hazards

  • Primary Hazard: Blast overpressure — the shock wave produced by 8 oz of TNT in enclosed spaces exceeds the overpressure of fragmentation grenades and is the primary casualty mechanism
  • Secondary Hazard: While designed as a minimum-fragmentation grenade, secondary missiles including fragments of the fuze assembly, body material, and environmental debris (rocks, gravel, wood, glass) can be projected as far as 200 meters from the detonation point
  • Effective Casualty Radius (open area): 2 meters
  • Secondary Fragment Hazard: Up to 200 meters
  • Health Hazard: The Mk3A2’s exterior coating contained up to 50% asbestos, which is hazardous when inhaled. This was the primary reason for the grenade’s withdrawal from U.S. service in 1975.
  • UXO Considerations:
    • The fiberboard body degrades relatively quickly compared to metal-bodied grenades, but the TNT filler and fuze assembly remain dangerous
    • Waterlogged specimens may have compromised fuze reliability but the TNT fill can remain functional
    • The fuze assembly (M206 series) contains metallic components that can be detected
  • Environmental Degradation: The fiberboard body is susceptible to moisture damage over time. In wet environments, the body may deteriorate while the TNT fill and detonator remain hazardous.

6. Key Identification Features

  • Shape: Cylindrical
  • Body Material: Waterproof asphalt-impregnated fiberboard
  • Body Construction: Two-piece cylinder secured in the middle with a wide black band
  • Fuze Well: Directly threaded in the center of the top cap
  • Color: Black body
  • Markings: Yellow lettering printed directly on the body surface reading “GRENADE HAND — OFFENSIVE MK3A2 — TNT”
  • Weight: 15.6 ounces (0.96 lbs / ~435 grams)
  • Dimensions: Cylindrical, approximately the size of a standard packing tube
  • Fuze: M206A1 or M206A2 (smokeless- and noiseless-burning, 5-second delay)
  • Safety Clip: May or may not be present depending on production date
  • Distinctive Features:
    • The two-half fiberboard body with the black band in the middle is the most recognizable feature
    • Yellow lettering on black body
    • Non-metallic body construction (unlike most other grenades)

7. Fuzing Mechanisms

  • Fuze Type: M206A1 or M206A2 — pyrotechnic delay, striker-release
  • Earlier Fuzes: The Mk3A2 originally used the M6A4D fuze (4–5 second delay), later replaced by the M206 series
  • Arming Sequence:
    1. Remove the safety clip (if present)
    2. Straighten the cotter pin ends and pull the safety pin
    3. Release the safety lever upon throwing
    4. The spring-loaded striker impacts the percussion primer
    5. The primer ignites the smokeless, noiseless pyrotechnic delay column
  • Delay Time: Approximately 4–5 seconds (M6A4D) or 5 seconds (M206 series)
  • Detonation: The delay column burns through to a detonator that initiates the 8 oz TNT main charge
  • Key Feature of M206 Series: Smokeless and noiseless burning — unlike earlier fuzes, the M206 does not produce visible smoke or audible hissing during the delay, reducing the chance of enemy detection and evasion
  • Safety Mechanisms:
    • Safety pin with pull ring
    • Safety clip (on later production models)
    • Spring-loaded safety lever

8. History of Development and Use

The Mk3 series traces its origins to March 1918, during World War I, when U.S. forces needed a grenade that could clear trenches, bunkers, and enclosed positions without producing fragmentation dangerous to the assaulting troops. The original Mk3 featured a simple cardboard body and the Mk.II fuze.

The Mk3A1 introduced a water-resistant laminated paper body with die-cast metal caps and upgraded M6-series fuzes. The Mk3A2, the final and most widely known variant, further improved the design with a waterproof asphalt-impregnated fiberboard body and an 8 oz TNT charge — a significant increase from the Mk3’s original fill weight.

The Mk3A2 saw extensive use during World War II, the Korean War, and the Vietnam War. It was valued for clearing bunkers, tunnels, buildings, and other enclosed spaces where the blast overpressure effect was more effective than fragmentation. U.S. special operations forces also used it for demolition tasks and improvised applications.

The grenade was formally withdrawn from U.S. Army service in 1975 after it was determined that the exterior coating contained up to 50% asbestos, posing a health hazard to troops handling the grenades. This left the M67 fragmentation grenade as the only anti-personnel grenade in the U.S. inventory for several decades, until the M111 Offensive Hand Grenade was adopted in 2021 to fill the concussion/blast role.

The Enhanced Tactical Multi-Purpose (ET-MP) grenade is currently in development to combine both fragmentation and blast overpressure effects in a single weapon with a selectable switch.

9. Technical Specifications

ParameterValue
Body ShapeCylindrical
Body MaterialWaterproof asphalt-impregnated fiberboard
Total Weight15.6 oz (0.96 lbs / ~435 g)
Explosive FillTNT
Fill Weight8 oz (~227 g)
FuzeM206A1 or M206A2
Fuze Delay~5 seconds
Effective Casualty Radius (open)2 m
Secondary Fragment RangeUp to 200 m
Throwing Range~40 m (average soldier)
Safety ClipYes (later production)
PackingCrates of 30 grenades in storage tubes

10. Frequently Asked Questions

Q: Why is the Mk3A2 called an “offensive” grenade rather than a “defensive” grenade? A: The distinction relates to fragmentation hazard. A defensive grenade (like the M67) produces lethal fragmentation that extends beyond the throwing range, meaning the thrower must be behind cover. An offensive grenade like the Mk3A2 produces minimal fragmentation, relying instead on blast overpressure. This allows an assaulting soldier to throw the grenade and continue advancing without needing to take cover from their own weapon — hence “offensive.”

Q: Why was the Mk3A2 withdrawn from service? A: The Mk3A2 was withdrawn from U.S. Army service in 1975 because its exterior coating contained up to 50% asbestos. Asbestos fibers, when inhaled, can cause serious health conditions including mesothelioma and asbestosis. The health risk to troops handling the grenades during storage, transport, and use was deemed unacceptable.

Q: How effective is the Mk3A2 in open areas versus enclosed spaces? A: In open areas, the Mk3A2 has an effective casualty radius of only 2 meters — quite limited compared to fragmentation grenades. However, in enclosed spaces (bunkers, rooms, tunnels), the blast overpressure reverberates off walls and ceilings, dramatically amplifying its effect. The overpressure produced by the Mk3A2 in enclosed areas actually exceeds that of fragmentation grenades, making it the weapon of choice for clearing fortified positions.

Q: What replaced the Mk3A2 in the U.S. inventory? A: For over four decades after the Mk3A2’s withdrawal in 1975, the U.S. Army had no dedicated concussion/blast grenade. The M67 fragmentation grenade was the sole anti-personnel grenade in the inventory. In 2021, the U.S. Army adopted the M111 Offensive Hand Grenade to fill this gap. The Enhanced Tactical Multi-Purpose (ET-MP) grenade is also being developed to combine fragmentation and blast effects in a single weapon.

Q: What is the difference between the M206 fuze and the older M6-series fuzes? A: The M206 series fuze features smokeless and noiseless burning of the pyrotechnic delay column, meaning there is no visible smoke emission or audible hissing sound during the delay period. Older M6-series fuzes produced detectable smoke and sound during the delay burn, potentially alerting the enemy. The M206 also provides a consistent 5-second delay.

Q: Could the Mk3A2 be used for demolition tasks? A: Yes. With 8 ounces of TNT, the Mk3A2 was frequently employed for light demolition, breaching, and destruction tasks in the field. Soldiers sometimes constructed field-expedient blast devices using blocks of TNT with grenade fuzes, essentially replicating the Mk3A2’s function.

Q: How can the Mk3A2 be distinguished from other cylindrical grenades? A: The Mk3A2 is identified by its black fiberboard body constructed in two halves secured by a wide black band in the middle, with yellow lettering reading “GRENADE HAND — OFFENSIVE MK3A2 — TNT” printed directly on the surface. The non-metallic body is a key distinguishing feature from metal-bodied smoke and incendiary grenades of similar size.

Q: Are there any UXO risks specific to the Mk3A2? A: The fiberboard body degrades faster than metal-bodied ordnance, especially in wet environments. However, the TNT fill is highly stable and can remain functional for many decades. The metallic fuze components (M206 series) and the TNT charge represent the primary hazard in any UXO encounter. A deteriorated Mk3A2 body may expose the TNT fill directly to the environment, creating both an explosive and a toxic exposure hazard.