M374A2 81mm High-Explosive Mortar

1. Overview

The M374A2 is a United States 81mm high-explosive (HE) mortar cartridge that served as a primary indirect fire support munition for U.S. Army and Marine Corps infantry battalions. This round represents a mature, combat-proven design that provided effective fragmentation effects against personnel and light materiel targets. The M374 series was a workhorse of American mortar units from the Vietnam era through the 1990s before being superseded by improved designs.

2. Country/Bloc of Origin

  • Country of Origin: United States of America
  • Development Period: 1960s, with the A2 variant incorporating incremental improvements
  • Manufacturer: Produced by multiple U.S. defense contractors under government contract
  • International Distribution: Widely exported to NATO allies and U.S. security partners; encountered globally
  • Current Status: Largely superseded by M821/M889 series in U.S. service; remains in use with allied and partner nations; significant quantities exist in legacy stockpiles worldwide

3. Ordnance Class

  • Type: Mortar cartridge (complete round)
  • Primary Role: High-explosive anti-personnel and light materiel destruction
  • Delivery Method: Muzzle-loaded, drop-fired from M29A1, M252, and other 81mm smoothbore infantry mortars
  • Category: Conventional indirect-fire munition

4. Ordnance Family / Nomenclature

  • Official Designation: Cartridge, 81mm, HE, M374A2
  • NSN: 1310-00-028-5251 (representative; may vary by lot)
  • DODIC: C624
  • Family Variants:
    • M374 – Original production model
    • M374A1 – First improvement with enhanced fuze compatibility
    • M374A2 – Current variant with improved propellant configuration
    • M374A3 – Limited production variant
  • Related Training Rounds:
    • M375 series – Practice rounds simulating M374 ballistics
  • Replacement: Superseded by M821 and M889 in U.S. service

5. Hazards

Primary Hazards

  • Blast Effect: High-explosive detonation produces lethal overpressure; danger close radius applies for friendly troops
  • Fragmentation: Cast iron body generates approximately 500+ fragments at high velocity; primary casualty mechanism
  • Effective Casualty Radius: Approximately 35 meters for personnel in the open
  • Bursting Radius: Fragments may travel 200+ meters with decreasing lethality

Sensitivity Considerations

  • Fuze Sensitivity: Point-detonating fuzes (M525 series) are impact-sensitive when armed
  • Propellant Hazards: Increment charges contain nitrocellulose-based propellant; flammable and may be unstable if deteriorated
  • Mechanical Safety: Multi-option fuzes have additional electronic components that may malfunction

UXO Considerations

  • Duds encountered in training areas and former conflict zones present extreme hazard
  • M525-series fuzes may be in armed state; piezoelectric elements can retain sensitivity
  • Rounds buried in soil may corrode, potentially exposing explosive fill
  • Do not disturb, move, or attempt to disarm

6. Key Identification Features

Physical Characteristics

  • Length: Approximately 490mm (19.3 inches) overall
  • Diameter: 81mm (3.19 inches) body
  • Weight: 4.24 kg (9.35 lbs) complete round
  • Body Shape: Streamlined ogival body with boattail base transitioning to tail boom

Visual Identification

  • Body Color: Olive drab with yellow band(s) indicating HE fill
  • Markings:
    • Nomenclature stenciled in yellow: “CART 81MM HE M374A2”
    • Lot number, year of manufacture, loading facility code
    • Ammunition Data Card (ADC) information
  • Fuze Well: Threaded nose well (2-inch, 12 threads per inch) for M525-series fuzes
  • Tail Assembly: Removable fin assembly with 8 canted fins; propellant increments attach to tail boom

Distinguishing Features from Similar Rounds

  • Yellow band identifies HE (versus white for illumination, red for WP)
  • Cast iron body distinguishes from later steel-bodied M821
  • Heavier than M821 series due to body construction
  • Fin assembly configuration specific to M374 series

7. Fuzing Mechanisms

Primary Fuze

  • M525 Series Point-Detonating Fuze: Standard fuze for M374A2
    • M525A1/A2: Multi-option fuze with superquick (SQ), near-surface burst (NSB), and delay (DLY) settings
    • Selector allows gunner to choose function based on target requirements

Fuze Characteristics

  • Arming Method: Setback forces during launch initiate arming sequence; spin imparted by canted fins completes arming
  • Arming Distance: Minimum 12-25 meters from muzzle (varies by fuze variant)
  • Function Options:
    • Superquick (SQ): Instantaneous detonation on contact; maximum fragmentation effect
    • Near-Surface Burst (NSB): Airburst approximately 1-2 meters above ground; enhanced anti-personnel effect
    • Delay (DLY): Short delay for penetration of light cover before detonation
  • Safety Features: Bore-riding pin, setback-actuated slider, spring-loaded rotor

Arming Sequence

  1. Round dropped into mortar tube
  2. Firing pin strikes primer; propellant ignites
  3. Setback forces release bore-riding safety pin
  4. Spin arms piezoelectric element
  5. After safe separation distance, fuze fully armed
  6. Upon target impact, piezoelectric crystal generates charge, initiating detonation

8. History of Development and Use

Development Context

The M374 was developed during the 1960s as part of U.S. efforts to modernize infantry fire support capabilities. The 81mm mortar, replacing the older 60mm as the standard battalion-level indirect fire weapon, required a new family of ammunition optimized for the M29A1 mortar system.

Evolution

  • M374: Original production variant fielded in mid-1960s
  • M374A1: Incorporated improvements to fuze compatibility and propellant configuration
  • M374A2: Enhanced safety features and improved increment charge design; became standard production model

Operational History

  • Vietnam War (1965-1975): Extensively employed by U.S. forces; proved effective against personnel in dense vegetation
  • Grenada (1983): Used during Operation Urgent Fury
  • Panama (1989): Employed during Operation Just Cause
  • Gulf War (1991): Final major U.S. combat use before replacement by M821

Transition to Modern Ammunition

During the 1990s, the U.S. military transitioned from the M374 to the M821 and M889 series, which offered improved fragmentation patterns, insensitive munitions (IM) characteristics, and compatibility with modern digital fire control systems. The M374 series was phased out of U.S. active stockpiles but remains in service with foreign militaries and may be encountered in legacy ammunition stocks.

9. Technical Specifications

SpecificationValue
Caliber81mm
Overall Length490mm (19.3 in)
Body Length287mm (11.3 in)
Total Weight4.24 kg (9.35 lbs)
Body MaterialCast iron
Explosive FillComposition B
Explosive Weight680g (1.5 lbs)
Muzzle Velocity100-300 m/s (varies with charge)
Maximum Range~4,800m (with M252 mortar)
Minimum Range~70m
Effective Casualty Radius~35m
Number of Charges0-4 increments + primary cartridge
Operating Temperature-40°F to +125°F (-40°C to +52°C)

10. Frequently Asked Questions

Q: What is the primary difference between the M374A2 and its replacement, the M821? A: The M821 features a steel body (versus cast iron in the M374A2), resulting in a more consistent fragmentation pattern and reduced weight. The M821 also incorporates insensitive munitions (IM) features, making it more resistant to unintended detonation from fire or impact. Additionally, the M821’s design is optimized for the M252 mortar system and modern fire control equipment. From a tactical standpoint, both provide similar effects on target, but the M821 offers improved safety and reliability.

Q: How does the M525 fuze’s near-surface burst (NSB) setting work? A: The NSB setting uses a standoff probe extending from the fuze nose. When this probe contacts the ground, it triggers detonation while the main body of the round is still slightly above the surface (approximately 1-2 meters). This airburst effect maximizes fragmentation dispersion against personnel in the open or in shallow defilade, as fragments travel downward and outward rather than being absorbed by the ground.

Q: Why is the M374A2 body made of cast iron rather than steel? A: Cast iron was selected for ease of manufacturing and its fragmentation characteristics. Cast iron tends to fracture into irregular fragments upon detonation, which was considered effective for anti-personnel purposes. However, the fragmentation pattern is less consistent than that achieved with controlled-fragmentation steel bodies used in later designs. The cast iron construction also makes the M374A2 heavier than its successors.

Q: What propellant increment configuration is used with the M374A2? A: The M374A2 uses an M224 primary propellant cartridge (fixed in the tail boom) plus up to four M223 or equivalent increment charges that horseshoe-clip around the tail boom. Each additional increment increases range. Charge selection is based on target distance: Charge 0 (primary only) for close targets; Charges 1-4 for progressively greater distances. Unused increments must be removed from the round before firing.

Q: How can I identify whether an M374A2 round has been fired (dud) versus unfired? A: Key indicators of a fired but unfuzed (dud) round include: propellant residue or discoloration around the tail, primer that shows strike marks, fins that may be damaged or discolored from propellant gases, and the round being found in a position inconsistent with storage (e.g., embedded in ground, lying in a firing range impact area). However, all suspected ordnance—whether fired or unfired—must be treated as equally hazardous. Only qualified EOD personnel can make final determinations.

Q: What is the danger close distance for friendly troops when employing the M374A2? A: Danger close varies by service and tactical situation but is generally defined as the distance within which fragmentation poses significant risk to friendly personnel. For 81mm HE mortar rounds, danger close is typically 80-100 meters minimum. However, fragments may travel 200+ meters with diminishing lethality. Fire direction centers apply risk estimate distances based on target location, friendly positions, and acceptable risk levels.

Q: Can the M374A2 be used in the newer M252 mortar system? A: Yes, the M374A2 is ballistically compatible with the M252 81mm mortar, which replaced the M29A1 in U.S. service. The M252’s longer barrel provides slightly increased muzzle velocity and range with the same ammunition. However, the M374A2 was designed before the M252’s introduction and doesn’t take full advantage of the newer mortar’s capabilities. Fire tables specific to each mortar/ammunition combination must be used.

Q: Why are there still M374A2 rounds encountered worldwide if the U.S. phased them out? A: The M374 series was produced in very large quantities and widely exported. Many nations continue to use U.S.-pattern 81mm mortars and maintain stockpiles of compatible ammunition. Additionally, ammunition produced decades ago may still be in storage or may have been transferred through legitimate arms sales or conflict-zone capture/redistribution. In UXO contexts, M374 series rounds are frequently encountered in former U.S. training areas and in regions that experienced U.S. military operations or received U.S. military assistance.

This material is for educational and training purposes only. All ordnance should be considered dangerous until rendered safe by qualified EOD personnel. Never handle suspected unexploded ordnance. Report all UXO finds to appropriate military or civilian authorities.