US M301A3 81mm Illumination Mortar Cartridge

1. Overview

The M301A3 is a United States 81mm illumination mortar cartridge designed to provide battlefield illumination during night operations or low-visibility conditions. It is a fin-stabilized, muzzle-loaded round that deploys a parachute-suspended illuminant candle at altitude, lighting a wide area for approximately 60 seconds. The M301A3 represents the final major evolution of the M301-series illumination rounds and remains a standard-issue munition in U.S. mortar platoons.

⚠ SAFETY WARNING: All ordnance should be considered dangerous until proven safe by qualified personnel. Suspected unexploded ordnance should never be handled by untrained individuals and must be reported to military or law enforcement authorities immediately. This information is for educational and identification training purposes only.

2. Country/Bloc of Origin

  • Country: United States of America
  • Era of Development: Cold War period; the M301-series traces its lineage through the M301A1 and M301A2, with the A3 variant entering service in the late 1960s–early 1970s
  • Service Use: U.S. Army and U.S. Marine Corps
  • Production: Manufactured at various U.S. ammunition plants; cartridges from the early 1970s are commonly encountered

3. Ordnance Class

  • Type: Mortar cartridge (illumination)
  • Primary Role: Battlefield illumination, signaling, and target area marking
  • Delivery Method: Muzzle-loaded, indirect fire from 81mm mortar systems (M29, M29A1, M252)

4. Ordnance Family / Nomenclature

  • Official Designation: Cartridge, 81mm, Illuminating, M301A3
  • Related Variants:
    • M301A1 — Earlier version with gas check bourrelet grooves; compatible with M84 powder-train time fuze
    • M301A2 — Intermediate variant with a shorter tail fin assembly (57mm shorter than M301A3); uses M84 fuze; burst height of 400m
    • M301A3 — Current standard variant; uses M84A1 tungsten-ring mechanical-time fuze; burst height of 600m
  • Companion Rounds (M252 system):
    • M853/M853A1 — Newer visible illumination cartridge (600,000 candlepower, uses M772A1 MTSQ fuze)
    • M816 — Infrared illumination cartridge for night vision device enhancement
  • NATO Interchangeability: Designed for use in the M29-series and M252 mortar systems; the M252 is adapted from the British L16A2

5. Hazards

  • Primary Hazard Types:
    • Pyrotechnic/Thermal: The illuminant candle contains approximately 621g of pyrotechnic composition that burns at extremely high temperatures. Contact with the burning candle or falling canister can cause severe thermal burns and start fires
    • Blast: The expelling charge in the fuze generates sufficient force to separate the round body at altitude; propellant charges produce launch blast
    • Impact Hazard: The spent canister, parachute assembly, and body sections fall to earth after burnout and can cause injury or damage
  • Sensitivity: The M84A1 fuze contains a powder train and percussion primer — it is sensitive to impact on the firing pin. The ignition cartridge (M71A2-primed M66A1) is percussion-sensitive
  • Propellant Hazards: Uses M185 augmenting propellant charges (up to 8 increments) which are flammable and sensitive to flame
  • UXO Considerations:
    • Rounds that fail to function at altitude (“blinds”) may fall intact with an armed fuze and unburned pyrotechnic composition
    • The time fuze may have only partially functioned, leaving the expelling charge intact
    • Illuminant candles that fail to ignite represent a significant thermal hazard if subsequently disturbed
    • Body sections and canister components are commonly found on training ranges

6. Key Identification Features

  • Overall Length: Approximately 25 inches (635mm) complete round
  • Caliber: 81mm (3.19 inches)
  • Shape: Cylindrical body tube with a conical ogive (nose) fitted with a prominent time fuze; tapered tail cone leading to a finned tail assembly
  • Color and Markings:
    • Current production: Painted white with black markings (standard U.S. color code for illumination rounds)
    • Vietnam-era production: May be found in overall olive drab (OD green) with a white band near the fuze (combat-era paint scheme)
    • Markings include lot number, date of manufacture, round designation “M301A3,” and fuze data
  • Fuze: The M84A1 fuze is a prominent brass-colored mechanical time fuze mounted on the nose, with a graduated time ring (0–25 seconds) and raised bosses at 5-second intervals; the “S” (safe) position and “SET” indicator are marked
  • Tail Assembly: Aluminum fin assembly with canted fins (for spin stabilization); fin shaft houses the M66A1 ignition cartridge
  • Material Composition: Steel body tube, aluminum tail fin assembly, brass fuze components

7. Fuzing Mechanisms

  • Primary Fuze: M84A1 Mechanical Time Fuze (tungsten-ring, powder-train type)
    • Function: Single-purpose time fuze that initiates the expelling charge after a set delay
    • Time Setting: Adjustable from 0 to 25 seconds in 1-second intervals using fuze setter M25
    • Safety Features:
      • Safety wire must be removed prior to firing (provides safety during handling and transport)
      • The fuze body has a “SAFE” (S) setting position
      • Setting indicator rib marked “SET” on the adjustment ring
    • Arming Sequence: Upon firing, setback forces arm the fuze; the powder train burns for the set time duration, then ignites the expelling charge
    • Expelling Charge: Built into the fuze; upon functioning, it generates gas pressure that separates the front and rear body sections, ejecting the parachute and illuminant candle assembly
  • Ignition Cartridge: M71A2-primed M66A1 primary cartridge in the fin shaft base; percussion-initiated upon striking the mortar’s fixed firing pin
  • Propellant System: M185 augmenting charges (8 horseshoe-shaped increments fitted between the fins); minimum of 2 charges required for firing, maximum of 8
  • Self-Destruct/Self-Neutralization: None. The round relies entirely on the time fuze to function

8. History of Development and Use

The M301-series illumination cartridges were developed to provide organic battlefield illumination capability to U.S. infantry battalion mortar platoons. The requirement emerged from World War II and Korean War experience demonstrating the critical importance of night illumination for defensive operations and offensive maneuver.

The original M301A1 entered service with the M29 mortar system, which began replacing the older M1 mortar in 1952. The M301A1 used the M84 powder-train time fuze and had certain design features including gas check bourrelet grooves.

The M301A2 followed as an improved variant, retaining the M84 fuze but with a shorter tail fin assembly. It provided illumination over a 1,100–1,200 meter area for at least 60 seconds from a burst height of approximately 400 meters.

The M301A3 represented the definitive improvement, incorporating the upgraded M84A1 tungsten-ring mechanical time fuze (replacing the earlier brass-ring design), a longer tail fin assembly, and an increased burst height of 600 meters, illuminating a 1,200-meter area for at least 60 seconds. This variant was compatible with higher charge settings (up to 8 propellant charges versus the A2’s maximum of 4), significantly extending the usable range.

The M301A3 served extensively during the Vietnam War as the standard 81mm illumination round. Vietnam-era production featured olive drab coloring rather than the standard white paint scheme. The round saw widespread use in defensive perimeter illumination, ambush detection, and fire support coordination.

With the introduction of the M252 mortar in 1987 (replacing the M29), the M301A3 remained in the ammunition inventory alongside newer rounds. It has since been supplemented by the M853A1 visible illumination cartridge (which uses the M772A1 MTSQ fuze and is range-matched to M252 HE rounds) and the M816 infrared illumination cartridge for use with night vision equipment.

The M301A3 has been employed in every major U.S. military operation from Vietnam through the Global War on Terror. It remains classified as a standard-issue training and combat round.

  • Maximum Range: Approximately 3,440 yards (3,150m) with the M29; varies with the M252 system
  • Minimum Range: Approximately 110 yards (100m)

9. Technical Specifications

ParameterSpecification
Caliber81mm
Overall Length~25 inches (635mm)
Body MaterialSteel tube
Tail AssemblyAluminum, canted fins
FuzeM84A1 Mechanical Time (tungsten-ring)
Illuminant Fill~621g pyrotechnic composition
Illumination Duration≥60 seconds
Burst Height~600m (660 yards)
Illuminated Area~1,200m (1,300 yards) diameter
PropellantM66A1 primary cartridge + up to 8x M185 augmenting charges
Maximum Range~3,150m (3,440 yards) with M29
Minimum Range~100m (110 yards)
Packaging2-round containers, ready to fire

10. Frequently Asked Questions

Q: What is the difference between the M301A3 and the newer M853A1 illumination cartridge? A: The M301A3 uses the M84A1 mechanical time fuze and was originally designed for the M29 mortar, while the M853A1 uses the M772A1 MTSQ fuze and is specifically range-matched to the M252’s HE and smoke cartridges. The M853A1 provides 600,000 candlepower illumination and uses a different propelling charge system (M219). Both provide approximately 60 seconds of illumination, but the M853A1 offers improved ballistic compatibility with the M252 system.

Q: Why was the M301A3 painted olive drab during the Vietnam War instead of the standard white? A: White-painted ammunition would have been highly visible in a combat environment, compromising the concealment of ammunition supply points and mortar positions. Vietnam-era production used OD green paint with a white band near the fuze for tactical purposes. Current production standards specify white with black markings for clear identification of illumination rounds in mixed ammunition stocks.

Q: Can the M301A3 be fired from the M252 mortar? A: Yes. The M301A3 is listed as compatible ammunition for the M252 mortar system. The M301-series rounds are classified as “Standard B” ammunition for the M252. However, they are not ballistically matched to the M252’s HE rounds like the newer M853A1 is, which means fire direction calculations require separate firing tables.

Q: What happens if the time fuze malfunctions and the round impacts the ground intact? A: A round that fails to function at altitude (“blind”) will fall to earth with an armed expelling charge and unburned pyrotechnic candle. This represents a dual hazard — the fuze’s expelling charge may still be sensitive to disturbance, and the pyrotechnic illuminant composition is a significant thermal/fire hazard. Such items should be treated as UXO and handled only by qualified EOD personnel.

Q: Why does the fuze require a safety wire that must be removed before firing? A: The safety wire physically prevents the fuze’s internal mechanisms from functioning during handling, transport, and storage. Removing the wire just before firing is a critical step in the preparation sequence. If the wire is left in place, the fuze will not function and the round will not illuminate. Conversely, a round with the safety wire removed should be treated with extreme caution.

Q: What are the falling hazards associated with illumination rounds? A: After the illuminant candle burns out (or during the deployment), several components fall to earth: the separated front and rear body sections, the burned-out canister, and the parachute. These metal components can cause injury or damage upon impact. Troops are trained to maintain awareness of overhead illumination rounds and avoid positioning directly beneath them. On training ranges, these falling components constitute significant debris hazards.

Q: How does the M301A3 compare to the M816 infrared illumination cartridge? A: The M301A3 produces visible white light, observable to the naked eye, while the M816 produces infrared illumination visible only through night vision devices. The M816 allows illumination of target areas without alerting the enemy to the presence of illumination support. The M816 uses the M772 MTSQ fuze and is designed exclusively for the M252 mortar system.

Q: What is the minimum number of propellant charges required to fire the M301A3? A: The M301A3 requires a minimum of two propellant charges. It can be fired with up to eight M185 augmenting charges. The number of charges selected determines range and, in conjunction with the tube elevation, the burst height and area illuminated. Firing with fewer than two charges is prohibited as it may result in a short round or bore obstruction.