US M375 81mm Bursting Smoke (WP) Mortar Cartridge

1. Overview

The M375 is a United States 81mm bursting white phosphorus (WP) smoke mortar cartridge designed for screening, signaling, marking, and incendiary effects. The round functions by detonating a small burster charge upon impact, which ruptures the body and disperses the white phosphorus filler. Upon contact with air, the WP ignites spontaneously, producing a dense white smoke screen and significant thermal effects. The M375-series has been a standard companion round to the M374-series HE cartridge throughout U.S. 81mm mortar service.

⚠ SAFETY WARNING: All ordnance should be considered dangerous until proven safe by qualified personnel. White phosphorus munitions present extreme thermal, toxic, and chemical hazards. 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 M375 was developed as the smoke companion to the M374 HE cartridge family
  • Service Use: U.S. Army and U.S. Marine Corps
  • International Note: The M375A3 is interchangeable with the British L19 smoke round; Iranian copies also exist under the M375 designation (based on Israeli M61 design)

3. Ordnance Class

  • Type: Mortar cartridge (bursting smoke/WP)
  • Primary Role: Screening smoke, casualty production, incendiary effects, signaling, and target marking
  • Delivery Method: Muzzle-loaded, indirect fire from 81mm mortar systems (M29, M29A1, M252)

4. Ordnance Family / Nomenclature

  • Official Designation: Cartridge, 81mm, Smoke, WP, M375
  • Related Variants:
    • M375 — Original version; companion to the M374 HE
    • M375A1 — First modification
    • M375A2 — Improved version; companion to the M374A2 HE; used with M29-series mortars
    • M375A3 — Current standard; companion to the M374A3 HE; ballistically similar to the M374A3; interchangeable with British L19
  • Companion HE Rounds: M374, M374A2, M374A3 (each smoke variant is ballistically matched to its corresponding HE version)
  • Related Smoke Rounds (M252 system):
    • M819 — Red phosphorus (RP) smoke cartridge (produces white smoke without the hazards of WP; modified to prevent phosphine out-gassing in storage)
  • Fuze Compatibility: Point-detonating (PD) or proximity fuzes depending on variant

5. Hazards

  • Primary Hazard Types:
    • Chemical/Thermal: White phosphorus ignites spontaneously on contact with air, burning at approximately 800°C (1,472°F). WP produces severe, deep chemical burns that are extremely difficult to treat. Burning WP particles can embed in flesh and continue burning
    • Toxic Fumes: WP combustion produces dense phosphorus pentoxide smoke (P₄O₁₀), which is toxic when inhaled in concentration. Phosphine gas may also be present
    • Blast: The RDX burster charge produces a blast effect sufficient to rupture the body and disperse the WP filler
    • Fragmentation: Body rupture produces metal fragments, though fragmentation is not the primary kill mechanism
  • Sensitivity:
    • The WP filler is pyrophoric — it will ignite spontaneously if the body is breached and the filler is exposed to air
    • The PD fuze is sensitive to impact
    • The RDX burster charge is a secondary explosive sensitive to detonation
  • Environmental Hazards: WP can contaminate soil and water. Partially burned WP can reignite when disturbed or when water evaporates from it
  • UXO Considerations:
    • Extremely hazardous as UXO. A dud M375 round contains both an armed fuze and an intact WP filler. Any disturbance may initiate the burster or rupture the body, releasing burning WP
    • WP rounds that have partially functioned may leave scattered chunks of unburned phosphorus on the ground that reignite when disturbed or dried
    • WP UXO should be approached with extreme caution; the standard RSP often involves destruction in place
    • Leaking WP from corroded or damaged rounds produces spontaneous ignition and toxic smoke

6. Key Identification Features

  • Caliber: 81mm (3.19 inches)
  • Shape: Cylindrical projectile body similar to the companion HE round, with a conical ogive and threaded nose for fuze; externally threaded base for fin assembly attachment
  • Color and Markings:
    • Painted light green with red markings and one narrow yellow band (U.S. standard color code for WP smoke munitions)
    • Light green body = smoke; red markings = special filler hazard; yellow band = WP-specific indicator
    • Markings include lot number, date, model designation, and filler type
  • Fuze: Point-detonating fuze mounted in the nose (M935 PD fuze or M734 multi-option fuze depending on variant and era)
  • Tail Assembly: Finned tail assembly with canted fins (5-degree cant for spin stabilization); ignition cartridge in the fin shaft base
  • Burster: A burster casing containing RDX is press-fitted into the adapter in the nose, running axially through the center of the projectile
  • Material: Steel body, aluminum fin assembly

7. Fuzing Mechanisms

  • Primary Fuze Options:
    • M935 Point-Detonating (PD) Fuze — Standard impact fuze; detonates on contact with the ground or target
    • M734 Multi-Option Fuze (MOF) — Offers proximity burst (PRX), near-surface burst (NSB), impact (IM), and delay (DLY) settings; no tools required to set; automatically defaults to the next lower setting if primary mode fails
    • Proximity fuzes may also be used depending on tactical requirements
  • Arming Sequence:
    • Upon firing, setback and spin forces arm the fuze
    • The M734 uses no safety pins or wires, reducing preparation time
    • The M935 arms through setback force
  • Functioning: Upon fuze initiation, the detonation impulse passes to the RDX burster charge running through the center of the projectile. The burster detonates, rupturing the projectile body and dispersing the WP filler into the air where it spontaneously ignites
  • Propelling Charge: M205 propelling charge system (for M375A3); consists of ignition cartridge and removable propellant increments fitted around the fin shaft
  • Self-Destruct/Self-Neutralization: None

8. History of Development and Use

The M375-series WP smoke mortar cartridge was developed as the organic smoke screening and marking capability for U.S. 81mm mortar platoons. The requirement for effective smoke rounds at the battalion level dates back to World War II, where smoke was critical for concealing troop movements, marking targets for air support, and providing signaling.

The original M375 was developed alongside the M374 HE cartridge, sharing the same basic body design but substituting WP filler for the HE charge and incorporating a central burster tube. Each subsequent variant (M375A1, M375A2, M375A3) was developed in parallel with its companion HE variant to maintain ballistic matching — a critical requirement for fire direction centers that need to transition between HE and smoke missions without extensive recalculation.

The M375A3, the current standard variant, is ballistically similar to the M374A3 HE cartridge and is interchangeable with the British L19 smoke round, reflecting the NATO standardization achieved when the U.S. adopted the British-designed M252 mortar system in 1987.

WP smoke rounds have been employed in every major U.S. military conflict from Korea through the Global War on Terror. Their use has been controversial due to the severe injury potential of white phosphorus. The development of the M819 red phosphorus (RP) smoke cartridge for the M252 system offered an alternative that produces equivalent smoke screening effects without the extreme incendiary hazards of WP, and was specifically modified to prevent phosphine out-gassing during storage.

The M375-series remains in the U.S. ammunition inventory and continues to be used for training and operational purposes. Its dual-role capability (smoke and incendiary/casualty-producing effects) makes it a versatile asset for mortar platoons.

9. Technical Specifications

ParameterSpecification
Caliber81mm
Body MaterialSteel
Tail AssemblyAluminum, fins canted at 5°
FillerWhite phosphorus (WP)
BursterRDX in press-fitted burster casing
Fuze OptionsM935 PD, M734 MOF, or proximity
Propelling ChargeM205 charge system
Color CodeLight green body, red markings, one narrow yellow band
Ballistic MatchM374A3 HE cartridge
Interchangeable WithBritish L19 smoke round (M375A3 variant)

10. Frequently Asked Questions

Q: Why is white phosphorus used instead of other smoke-producing agents? A: White phosphorus offers several tactical advantages: it ignites spontaneously on contact with air (requiring no separate ignition mechanism), produces an extremely dense white smoke screen very rapidly, burns at high temperatures providing incendiary effects, and can produce casualties through thermal and chemical burns. However, its extreme hazard profile has led to the development of alternatives like the M819 red phosphorus round, which provides comparable screening smoke without the severe incendiary effects.

Q: How can you distinguish an M375 WP round from an M374 HE round in the field? A: The color coding is the primary identifier. The M375 WP round is painted light green with red markings and a yellow band, while the M374 HE round is painted olive drab with yellow markings. The body shapes are nearly identical since they share the same basic projectile design, making color coding essential for correct identification. Always verify markings — mixed-up ammunition can have catastrophic consequences.

Q: What makes WP rounds particularly hazardous as UXO? A: WP UXO presents a compound hazard. The round contains both a conventional explosive component (the RDX burster and armed fuze) and a pyrophoric chemical agent (white phosphorus). Any breach of the body — from corrosion, physical damage, or disturbance — can expose the WP to air, causing spontaneous ignition and toxic smoke. Additionally, partially functioning rounds may scatter unburned WP particles that reignite unpredictably when disturbed or when moisture evaporates. The standard approach for WP UXO is typically destruction in place.

Q: What is the difference between the M375 WP round and the M819 RP round? A: The M819 uses red phosphorus (RP) instead of white phosphorus. Red phosphorus produces white smoke on contact with air similar to WP but does not ignite spontaneously with the same intensity, making it significantly less hazardous as an incendiary agent. The M819 was specifically modified to prevent phosphine out-gassing during storage, a problem associated with phosphorus-based munitions. The M819 uses the M218 propelling charge and is painted light green with black markings and a brown band.

Q: Can the M734 multi-option fuze be set to proximity mode on a WP round? A: The M734 fuze can technically be set to any of its four modes (proximity, near-surface burst, impact, delay) on the M375. However, tactical employment typically uses impact or delay settings for smoke rounds, as the intent is to place the smoke screen on or very near the ground. Proximity burst of a WP round would scatter burning phosphorus over a wider area at altitude, which may be tactically undesirable for screening but could be employed for other effects.

Q: What is the purpose of the narrow yellow band on the M375? A: The yellow band specifically indicates that the round contains white phosphorus filler. This is a critical safety marking that distinguishes WP rounds from other smoke types. In U.S. ammunition color coding, yellow markings generally indicate the presence of a hazardous chemical or special filler. The combination of light green body (smoke), red markings (special hazard), and yellow band (WP) provides a three-layer identification system.

Q: How does the burster mechanism work to disperse the WP? A: The RDX burster charge is contained in a casing that runs axially through the center of the projectile, press-fitted into the nose adapter. When the fuze detonates, it initiates the burster charge, which detonates along the length of the projectile body. This central detonation creates sufficient pressure to rupture the steel body outward, fragmenting it and dispersing the WP filler into the surrounding air. The WP ignites spontaneously upon exposure to oxygen, creating the characteristic dense white smoke and thermal effects.

Q: Why is the M375 ballistically matched to the M374 HE round? A: Ballistic matching allows mortar fire direction centers to quickly shift between HE and smoke missions using the same firing data (deflection and elevation settings) with only minor adjustments. If the rounds were not ballistically matched, each transition between HE and smoke would require complete recalculation of firing solutions, adding dangerous delays in time-critical fire missions. This is why each M375 variant was developed in parallel with its corresponding M374 HE variant.