M929 120mm Bursting Smoke (WP) Mortar Cartridge

Overview

The M929 is a 120mm bursting smoke mortar cartridge designed to produce rapid battlefield obscuration through the dispersion of white phosphorus (WP). This round provides U.S. Army mortar crews with an effective screening and marking capability, allowing commanders to conceal friendly movement, blind enemy positions, or designate targets for engagement by other weapons systems. The M929 represents a critical component of the U.S. 120mm mortar ammunition family, combining the destructive potential of white phosphorus with the extended range and firepower of heavy mortar systems.

Country/Bloc of Origin

  • Country: United States of America
  • Development Period: Late 1980s to early 1990s
  • Primary Developer: Martin Marietta Ordnance Systems (original contractor)
  • Current Production: General Dynamics Ordnance and Tactical Systems

The M929 was developed as part of the U.S. Army’s effort to field a complete family of ammunition for the newly adopted M120/M121 120mm mortar system, which entered service in 1991.

Ordnance Class

  • Type: Mortar cartridge (fin-stabilized)
  • Primary Role: Smoke/obscuration, target marking, incendiary
  • Secondary Role: Anti-personnel (incendiary effect)
  • Delivery Method: Indirect fire from M120 (towed) and M121 (vehicle-mounted) 120mm mortar systems

Ordnance Family/Nomenclature

  • Official Designation: Cartridge, 120mm Smoke WP, M929
  • Developmental Designation: XM929, XM929E1
  • Related Variants:
    • M929 (production model with M734A1 Multi-Option Fuze)
    • XM929 (developmental variant with M745 Point Detonating Fuze)
    • XM929E1 (improved developmental variant)
  • Associated Ammunition Family:
    • M933 HE Cartridge
    • M934 HE Cartridge (with multi-option fuze)
    • XM930/M930 Illumination Cartridge
    • M931 Full Range Training Cartridge

Hazards

Primary Hazards
  • Chemical/Thermal: White phosphorus burns at approximately 1,500°F (815°C) and will continue burning until completely consumed or deprived of oxygen
  • Fragmentation: Steel body fragments upon burster charge detonation
  • Toxic Fumes: WP smoke is toxic and can cause respiratory damage
Sensitivity Considerations
  • Impact Sensitive: Fuze is designed to function on impact; rough handling of fuzed rounds should be avoided
  • Heat Sensitive: WP filler is spontaneously combustible when exposed to air
Special Hazards
  • Persistent Burning: WP particles can embed in skin and continue burning; water does not extinguish WP
  • Re-ignition Risk: WP particles may re-ignite when removed from water and exposed to air
  • Secondary Fires: Capable of igniting combustible materials in target area
UXO Considerations
  • Unexploded M929 rounds pose extreme hazards due to WP filler
  • Damaged rounds may leak WP, creating immediate fire hazards
  • Never attempt to move or disturb suspected UXO containing WP
  • Minimum safe distance for EOD approach should account for potential WP dispersion

Key Identification Features

Physical Characteristics
  • Length: Approximately 27-28 inches (686-711 mm) complete cartridge
  • Diameter: 120mm (4.72 inches) body diameter
  • Weight: 13.65 kg (30 lb) complete cartridge
External Features
  • Body: 1045 wrought carbon steel construction
  • Shape: Streamlined ogive nose with cylindrical body tapering to fin assembly
  • Fin Assembly: Multi-fin stabilizer with flash holes for propellant ignition
  • Propellant Containers: Four horseshoe-shaped felt fiber containers around fin shaft
Color and Markings
  • Body Color: Light green (standard U.S. smoke ammunition color)
  • Markings: Yellow band and markings indicating WP content
  • Stenciling: Lot number, manufacturer code, date of manufacture
Distinctive Features
  • Fuze well at nose accepts M734A1 Multi-Option Fuze or M745 PD Fuze
  • Felt wedges impregnated with WP visible through body (internal)
  • Obturating ring assembly at base of projectile

Fuzing Mechanisms

Primary Fuze Options

M734A1 Multi-Option Fuze (Production Model)

  • Selectable modes: Proximity (PRX), Near Surface Burst (NSB), Impact (IMP), Delay (DLY)
  • Electronic proximity sensing for airburst capability
  • Battery-powered with setback arming

M745 Point Detonating Fuze (Developmental)

  • Impact or Delay mode only
  • Mechanical fuze with impact-activated firing mechanism
Arming Sequence
  1. Upon firing, setback forces arm the fuze
  2. Centrifugal forces (from spin imparted by fin drag) complete arming sequence
  3. Fuze functions at selected mode upon approaching/impacting target
Functioning
  • Fuze initiation triggers M86 Projectile Burster (Composition B explosive)
  • Burster charge ruptures steel body
  • WP-saturated felt wedges are dispersed
  • WP ignites spontaneously upon contact with air, generating dense white smoke

History of Development and Use

Development Timeline
  • Late 1980s: Development initiated to support M120 mortar system adoption
  • 1991: M120 mortar system enters U.S. Army service
  • 1990s: M929 achieves type classification and enters production
  • Present: Remains in active U.S. Army inventory
Operational Context

The M929 was developed to replace the aging 4.2-inch (107mm) mortar smoke capabilities with a more modern, longer-range system. The 120mm caliber was selected based on the Israeli Soltam K6 mortar system, which provided superior range and payload compared to the legacy 4.2-inch system.

Combat Employment
  • Screening: Creating smoke screens to conceal friendly movement during assault or withdrawal
  • Blinding: Obscuring enemy observation posts and weapon positions
  • Marking: Designating targets for air strikes or artillery engagement
  • Incendiary Effect: Secondary capability against combustible targets
Current Status
  • In active service with U.S. Army and allied nations
  • Produced by General Dynamics Ordnance and Tactical Systems
  • Continual improvements to fuzing and reliability

Technical Specifications

SpecificationValue
Caliber120mm
Complete Cartridge Weight13.65 kg (30 lb)
Body Material1045 Wrought Carbon Steel
FillerWhite Phosphorus (WP) impregnated felt wedges
Burster ChargeComposition B (M86 Projectile Burster)
Maximum Range~7,200 meters
Minimum Range~200 meters
Maximum Rate of Fire16 rounds/minute (first minute)
Sustained Rate of Fire4 rounds/minute
Compatible MortarsM120, M121
PackagingPA153 Fiber Container (individual); PA154 Metal Overpack (2 rounds)

Frequently Asked Questions

Q: What is the difference between the M929 and older WP smoke rounds like the M328 107mm? A: The M929 provides significantly greater range (over 7,000 meters versus approximately 5,500 meters for the M328), a heavier payload, and compatibility with the modern M734A1 multi-option fuze that allows for airburst, proximity, and impact modes. The 120mm system also offers better accuracy and a more robust logistics footprint aligned with NATO standards.

Q: Why does the M929 use felt wedges impregnated with WP rather than liquid WP filler? A: The felt wedge design provides more controlled dispersion of the WP upon detonation, creating a broader and more uniform smoke screen. It also improves safety during storage and handling, as the WP is bound within the felt material rather than existing as a free liquid that could leak from damaged rounds.

Q: Can the M929 be used in the airburst mode for maximum obscuration effect? A: Yes. When equipped with the M734A1 Multi-Option Fuze set to Proximity (PRX) or Near Surface Burst (NSB) mode, the round can be detonated above the ground, dispersing the WP wedges over a wider area and creating a more effective smoke screen more rapidly than ground-burst detonations.

Q: What is the difference between the M929 with M734A1 fuze and the XM929 with M745 fuze? A: The production M929 uses the M734A1 Multi-Option Fuze, which provides four selectable modes (proximity, near surface burst, impact, delay). The developmental XM929 used the simpler M745 Point Detonating Fuze, which only offered impact or delay modes. The M734A1 provides greater tactical flexibility.

Q: How does the M929 compare to HC (Hexachloroethane) smoke rounds? A: WP smoke rounds like the M929 produce smoke more rapidly and are more effective in damp conditions, but they have significant incendiary effects and create toxic fumes. HC smoke is slower to develop but is non-incendiary and less toxic. The M929’s dual screening/incendiary capability makes it preferred for many tactical situations despite its hazards.

Q: What are the primary safety concerns when handling M929 rounds? A: Key concerns include: (1) Never handle damaged rounds that may be leaking WP, (2) Store in approved facilities with proper ventilation, (3) Use only approved handling procedures for fuzed rounds, (4) Be aware that WP burns cannot be extinguished with water alone—submerge affected area and seek immediate medical attention, (5) Ensure proper ventilation when firing to avoid crew exposure to WP fumes.

Q: How is the M929 typically employed tactically in a mortar fire mission? A: The M929 is typically employed in “immediate smoke” missions to rapidly screen friendly movement or blind enemy positions. Fire missions may combine WP smoke with HE rounds—the WP provides obscuration while HE suppresses enemy forces. Standard doctrine calls for adjusting WP rounds between friendly and enemy forces, not directly on friendly positions due to the incendiary hazard.

SAFETY NOTICE: This document is for educational and training purposes. All ordnance should be treated as dangerous until rendered safe by qualified EOD personnel. Report any suspected unexploded ordnance to appropriate military or law enforcement authorities.