US M5 Pressure Release Firing Device

Overview

The M5 Pressure Release Firing Device is a mechanically-actuated initiator designed to function when downward pressure is removed from the device—the opposite operating principle of the M1 Pressure Firing Device. This “anti-lift” or “release” function makes the M5 particularly effective for protecting objects from being moved, creating booby traps under items, and establishing sophisticated area denial systems. The device is placed under an object while compressed, and any attempt to lift or remove that object releases the internal striker, initiating the attached explosive. This inverse activation method creates significant challenges for anyone attempting to clear an area, as simple avoidance or careful stepping is insufficient protection.

Country/Bloc of Origin

Country: United States of America
Developer: US Army Ordnance Corps
Time Period: World War II era development, with continued refinement
Service Branch: US Army, US Marine Corps, and special operations forces
International Use: Distributed to NATO allies; design concept widely adopted and adapted by other nations; the pressure-release principle has been incorporated into numerous foreign anti-lift devices

Ordnance Class

Type: Mechanical firing device / Pressure release-actuated initiator
Primary Role: Anti-lift triggering mechanism for booby traps, anti-handling devices, and demolition applications
Delivery Method: Hand-emplaced
Category: Pressure release firing device (single-function)
Function: Converts the release of mechanical pressure into percussion ignition of a detonator or igniter

Ordnance Family/Nomenclature

Official Designation: Firing Device, Pressure Release, M5
NSN (National Stock Number): 1375-00-690-3650
Related Family Members:
- M1/M1A1 Pressure Firing Device (fires when pressure applied)
- M3 Pull Firing Device (fires when tension applied)
- M142 Combination Firing Device (pressure and pull combination)
Complementary Use: Often employed in conjunction with M1 devices to create “dual-trap” configurations where both stepping on AND lifting objects causes detonation
Common Names: “Pressure Release,” “Anti-Lift Device,” “M5,” “Release Device”

Hazards

Primary Hazards

Anti-Handling Function: The pressure release mechanism makes the M5 extremely dangerous to any clearance attempt—lifting the covering object initiates detonation
Concealment Under Objects: The device is designed to be hidden under seemingly innocent items, making detection extremely difficult
No Visual Warning: When properly emplaced, the M5 is completely hidden from view

Sensitivity Factors

Release Threshold: Functions when approximately 5 pounds (2.3 kg) or more of load is removed
Pre-compression Requirement: Device must be placed under load to arm—the loaded state holds the striker in compression
Settling and Shifting: Ground settling, thermal expansion/contraction, or vibration may cause unintended release

Special Hazards

Catastrophic for Clearance Operations: Standard mine clearance techniques (probing, lifting objects) can trigger M5 devices
Combination Trap Potential: Frequently used in combination with pressure-activated devices—the object hiding the M5 may itself be booby-trapped with an M1
Vehicle Hazards: If placed under vehicle wheels or tracks, any movement may cause release
Environmental Triggering: Flooding, erosion, animal disturbance, or decay of covering objects can cause release over time

Kill/Danger Radius

Determined entirely by the attached explosive charge
Common employment with demolition blocks creates lethal fragmentation zones of 10-50+ meters

UXO Considerations

EXTREMELY DANGEROUS to approach or investigate
Do not touch, probe, or attempt to lift any object in the vicinity
Assume all disturbed items may conceal M5 devices
Evacuate to maximum safe distance and report to EOD
Special robotic or remote clearance techniques often required

Key Identification Features

Physical Characteristics

Length: Approximately 3.5 inches (89 mm) when uncompressed
Compressed Length: Approximately 2.5 inches (64 mm) under load
Diameter: Approximately 1.25 inches (32 mm) at base
Weight: Approximately 2.5 ounces (71 grams)
Material: Steel or brass body with internal steel spring and striker mechanism

Distinctive Features

Release Plate Assembly: Wide, flat top plate designed to accept downward load—this is the defining visual characteristic
Compression Spring Visible: When unloaded, the extended compression spring may be visible around the body
Three-Legged Base Stand: Some variants feature a tripod-style base for stability
Threaded Base: Standard US military thread pattern for detonator attachment
Color: Olive drab or natural metal finish (brass/steel)

Markings

“M5” designation stamped on body
“FIRING DEVICE PRESSURE RELEASE” or “PRESS REL” text
Manufacturer information
Lot number and date of manufacture
May include NSN on packaging

Armed vs. Safe Visual Indicators

Uncompressed (Safe/Unarmed): Release plate sits high, spring extended
Compressed (Armed): Release plate depressed flush or near-flush with body, spring hidden
CAUTION: A compressed M5 appears smaller and may be mistaken for other devices

Fuzing Mechanisms

Operating Principle (Inverse of Pressure Devices)

The M5 operates on the principle of pressure RELEASE rather than pressure APPLICATION:

Emplacement: The device is positioned with the release plate facing upward
Loading: A heavy object (rock, equipment, debris, weapon) is placed on the release plate, compressing the internal spring
Armed State: Under load, the compressed spring holds the striker mechanism cocked against a release sear
Safety Removed: The safety pin is withdrawn while the device is under load
Release Event: When the loading object is lifted or removed, the spring extends, releasing the striker
Percussion: The striker impacts the percussion primer
Initiation: Primer detonation initiates the attached detonator and main charge

Critical Distinction

M1 (Pressure): Stepping ON causes detonation
M5 (Pressure Release): Lifting OFF causes detonation

Arming Sequence

Select emplacement location
Thread the M5 onto the detonator holder or coupling base
Insert the detonator
Position the M5 with release plate upward
Place sufficient weight on the release plate to fully compress the spring
While weight is in place, remove the safety pin
Carefully withdraw hands and depart without disturbing the load
Camouflage the setup as appropriate

Safety Mechanisms

Positive Safety Pin: Prevents striker release regardless of spring state
Pre-Load Requirement: Device cannot function unless first loaded and then released (prevents function during transport)
Shipping Configuration: Spring uncompressed, safety pin installed

Disarming Complexity

Disarming requires reapplication of load BEFORE safety pin can be reinserted
In practice, attempting to disarm a suspected M5 is extremely hazardous
Standard procedure is destruction in place by EOD

History of Development and Use

Development Context

The M5 Pressure Release Firing Device was developed during World War II in response to tactical requirements for anti-lift and anti-handling devices. Military planners recognized that pressure-only devices (like the M1) could be defeated by careful movement and avoidance. A complementary device that functioned on lift/release would force enemies to contend with a more complex threat matrix.

Tactical Philosophy

The pressure release concept serves several tactical purposes:

Object Protection: Prevents enemy recovery of equipment, weapons, or supplies
Clearance Denial: Makes mine/booby trap clearance far more dangerous and time-consuming
Psychological Effect: Creates doubt and hesitation about touching ANY object
Combination Potential: When paired with pressure devices, creates nearly unsolvable tactical problems

Historical Employment

World War II:

Used in European and Pacific theaters
Employed to protect abandoned equipment from enemy recovery
Placed under bodies of fallen soldiers (a war crime tactic also used by enemy forces)
Concealed under debris in defensive positions

Korean War:

Continued use in defensive and delaying operations
Employed in perimeter protection

Vietnam War:

Extensive use in counter-infiltration operations
Base perimeter defense applications
Combined with M1 devices for complex trap configurations

Modern Conflicts:

Training use continues at military engineer schools
Concepts employed in various contingency operations
Understanding M5 function essential for EOD personnel encountering similar foreign devices

Adversary Employment

The pressure release concept has been adopted by militaries worldwide. US forces have encountered similar devices manufactured by:

Soviet Union/Russia
China
Various non-state actors using improvised pressure release mechanisms

Understanding the M5 provides foundation for recognizing all pressure release threats.

Current Status

In Service: Yes, remains in US military inventory
Production: Continues as required
Training Priority: Essential knowledge for combat engineers and EOD personnel
Historical Significance: Established doctrinal concepts for anti-handling device employment

Technical Specifications

Specification	Value
Overall Length (Uncompressed)	3.5 inches (89 mm)
Overall Length (Compressed)	2.5 inches (64 mm)
Body Diameter	1.25 inches (32 mm)
Release Plate Diameter	1.5 inches (38 mm)
Weight	2.5 oz (71 g)
Minimum Load to Compress	5 lbs (2.3 kg)
Release Threshold	Removal of approximately 5 lbs (2.3 kg)
Operating Temperature	-40°F to +160°F (-40°C to +71°C)
Thread Size	Standard military coupling base thread
Material	Steel or brass body, steel internal components
Shelf Life	10+ years in proper storage
Compatible Initiators	M7 non-electric blasting cap, special blasting caps

Operating Parameters

Optimal Loading: 10-50 lbs (4.5-23 kg) provides reliable arming without over-stressing mechanism
Maximum Loading: Do not exceed 100 lbs (45 kg) to prevent mechanism damage
Release Sensitivity: Functions with partial release—even slight lifting of load can trigger

Frequently Asked Questions

Q: Why would someone use an M5 instead of the simpler M1 pressure device? A: The M5 serves tactical purposes the M1 cannot. Pressure devices can be avoided by careful movement, but pressure release devices require the enemy to leave objects undisturbed—impossible when conducting clearance operations or attempting to recover equipment. The M5 is particularly valuable for protecting supplies, weapons, or bodies from enemy recovery, and for creating extreme hazards during mine clearance operations. The two devices complement each other when used in combination.

Q: What is a “dual-trap” or combination employment of M5 and M1 devices? A: A dual-trap involves placing an M5 under an object with an M1 on top or nearby. Stepping on or near the object triggers the M1. Attempting to lift the object (to check underneath or remove an apparent M1) triggers the M5. This creates an extremely difficult tactical problem—the target cannot safely step on, step around, or lift the object. Such configurations require remote destruction rather than manual clearance.

Q: How much weight is required to arm the M5? A: The M5 requires approximately 5 pounds (2.3 kg) minimum to fully compress the spring and enable arming. In practice, objects weighing 10-50 pounds provide optimal performance. Common loading objects include rocks, ammunition boxes, equipment cases, debris, weapons, or even bodies. The key is that the weight must be sufficient to fully depress the release plate.

Q: Can an M5 spontaneously fire without human intervention? A: Yes, this is a significant hazard. Any process that removes load from the release plate can cause function: erosion washing away supporting soil, animal activity disturbing the covering object, decay of organic concealment material, flooding and receding water, or even gradual settling of the ground. This makes M5-contaminated areas dangerous even without human entry and complicates long-term area clearance.

Q: How does weather affect M5 function? A: Temperature extremes can affect spring tension and mechanism function. Freezing moisture may temporarily increase the release threshold or (upon thawing) cause unpredictable sensitivity. Extreme heat may soften any plastic components and affect performance. Corrosion from moisture exposure may cause mechanism binding or spontaneous release. All weather-exposed M5 devices should be considered extremely unpredictable.

Q: What is the proper response upon discovering a suspected M5 emplacement? A: Do NOT touch, probe, lift, or disturb ANY objects in the area. The suspected M5 and its covering object must remain completely undisturbed. Mark the location with visible warnings from a safe distance, evacuate all personnel to maximum safe distance (minimum 300 meters for unknown charge sizes), and immediately notify EOD personnel. Standard clearance procedures may be inadequate—remote destruction is often the only safe option.

Q: Can the M5’s release plate be seen when properly emplaced? A: No, proper emplacement completely conceals the M5. The device is placed in a hole or depression with only the release plate above grade level, and the covering object sits directly on the plate. The covering object appears natural or deliberately placed but innocent. Nothing visible indicates the presence of the M5 underneath. This is why area search based on visual inspection alone cannot clear M5-contaminated zones.

Q: How is the M5 concept used in improvised explosive devices (IEDs)? A: The pressure release principle has been widely adopted by non-state actors using improvised devices. While not using the actual M5, insurgents and terrorists have created pressure release mechanisms using modified syringes, clothespins, spring-loaded strikers, and other improvised components. Understanding the M5’s operating principle is essential for recognizing these improvised variants. Any device that functions when lifted or relieved of weight employs the same concept.

Safety Notice

All ordnance, including firing devices, should be considered dangerous until rendered safe by qualified explosive ordnance disposal (EOD) personnel. Never handle, collect, or disturb suspected explosive devices or components. If you encounter a suspected time pencil or similar device, do not touch it—mark the location and contact military or law enforcement authorities immediately. This lesson is provided for educational and identification training purposes only.