US Model 308-6 Firing Device (Hamilton 308 Series Bomb Timer)

Overview

The Model 308-6 Firing Device, part of the Hamilton 308 series of remote firing devices, represents one of the most innovative and covert time-delay initiators developed for U.S. special operations forces. Developed by the Hamilton Watch Company under classified U.S. Navy contract in the 1960s, these devices were ingeniously designed to appear as ordinary military wristwatches while concealing electrical firing contacts capable of initiating explosive charges. The 308 series provided Navy SEALs and Underwater Demolition Teams with unprecedented “stand-off capability” for covert sabotage operations, particularly during the Vietnam War era. The designation “308-6” refers to a specific model within the broader 308 series of remote firing devices.

Country/Bloc of Origin

  • Country: United States of America
  • Service Branch: United States Navy (Naval Special Warfare)
  • Development Period: 1960s (Cold War era)
  • Manufacturer: Hamilton Watch Company, Lancaster, Pennsylvania
  • Development Location: China Lake Naval Weapons Center (development contract)
  • Classification Status: Originally classified; documents declassified in later years
  • Primary Users: U.S. Navy SEALs, Underwater Demolition Teams (UDT-11 and others)

Ordnance Class

  • Type: Electrical time-delay firing device disguised as wristwatch
  • Primary Role: Covert initiation of explosive charges with time delay up to 24 hours
  • Delivery Method: Worn or carried by operator; connected to charge at emplacement
  • Classification: Electrical firing device; watch-based timer
  • Application: Covert sabotage, limpet mine initiation, delayed-action demolition, special operations

Ordnance Family/Nomenclature

Official Designations

  • Series Designation: 308 Remote Firing Device Models
  • Specific Model: Model 308-6
  • Hamilton Internal Reference: 3801-308 Device
  • Common Names: “308 Bomb Timer,” “Hamilton Bomb Timer,” “SEAL Timer”

Related Models in 308 Series

The 308 series comprised multiple models with varying specifications:

  • Model 308-1 through 308-6: Different variants optimized for specific operational requirements
  • Variations may have included different time ranges, waterproofing levels, or output characteristics

Related Navy Timing Devices

  • MK 3 Demolition Clock: Earlier mechanical firing device
  • MK 12/13/15 Series: Standard Navy clockwork firing devices
  • MK 23 Mod 1: Later Navy demolition firing device

Reference Documentation

  • Declassified Navy contract documents
  • Hamilton Watch Company museum archives
  • Naval Special Warfare historical records

Hazards

Primary Hazards

  • Electrical Discharge Hazard: Device produces electrical output capable of initiating electric detonators
  • Concealed Danger: Disguised appearance makes recognition difficult
  • Timing Uncertainty: Pre-set timers may activate at any moment if armed
  • Secondary Explosion Hazard: Connected charges will detonate upon device activation

Handling Precautions

  • Treat as armed if time setting or status is unknown
  • Never connect to explosive charges until ready for emplacement
  • Verify electrical isolation before any handling or inspection
  • Do not attempt to open or modify the watch case
  • Exercise extreme caution with electrical contacts on case back

Electrical Hazards

  • Firing contacts on case back may carry charge even if watch appears non-functional
  • Residual electrical energy may be present in aged devices
  • Corroded contacts may behave unpredictably

UXO/Legacy Considerations

  • Devices may appear as ordinary watches—positive identification required
  • Internal batteries may still hold charge after decades
  • Vintage devices encountered as collectibles require verification of inert status
  • Original devices were largely destroyed in operational use—survivors are rare

Key Identification Features

External Appearance

  • Disguised as Wristwatch: Intentionally resembles standard 24-hour military field watch
  • Case Style: Metal case typical of 1960s military timepieces
  • Dial: 24-hour format; some variants featured open-worked (skeletonized) dial showing movement
  • Watch Band: Standard military-style strap or bracelet

Critical Distinguishing Features

Case Back

  • Plastic Covering: Conceals electrical firing contacts
  • Two Electrical Contacts: Hidden beneath plastic cover on case back
  • Contact Purpose: Provide electrical output when hands reach 24:00 position

Dial Indicators

  • 24-Hour Scale: Full 24-hour time display
  • Time Indication: Shows countdown to firing time

Dimensions

  • Size: Comparable to standard men’s military wristwatch of the era
  • Weight: Slightly heavier than standard watch due to internal modifications

Markings

  • Hamilton Watch Company markings (may or may not be present depending on variant)
  • Military contract markings may be absent for covert purposes
  • Serial numbers may have been removed or not applied

Fuzing Mechanisms

Operating Principle

The Model 308-6 utilizes the precision of a quality watch movement to provide accurate time-delay initiation:

  1. Timer Setting: Operator sets the watch hands to desired firing time (maximum 24 hours)
  2. Electrical Generation: As the watch runs, it counts down to the set time
  3. Contact Closure: When the hands reach the 24:00 (0:00) position, internal mechanism closes electrical circuit
  4. Output: Electrical current flows between the two contacts on the case back
  5. Initiation: Connected electric blasting cap or detonator is fired

Time Delay Range

  • Maximum Delay: Approximately 24 hours (full watch cycle)
  • Minimum Delay: Variable depending on operational requirements
  • Accuracy: Comparable to quality mechanical watch movement

Electrical Output

  • Sufficient current and voltage to reliably initiate standard military electric detonators
  • Contact design ensures reliable connection when wired to charge

Arming Sequence

  1. Wire firing device to electric detonator/charge
  2. Set watch to desired time before firing
  3. Emplace device and charge on target
  4. Evacuate area before timer completion

Safety Considerations

  • No external safe/arm indicator—operator must track time setting
  • Once set and connected, no abort mechanism
  • Watch continues running until firing or until mechanism stops

History of Development and Use

Development Background

The Hamilton Watch Company had a long history of military timekeeping, dating to World War I. During WWII, Hamilton produced over one million military watches and 10,000 marine chronometers for the U.S. Navy, earning five Army-Navy “E” Awards for manufacturing excellence. This expertise made Hamilton the natural choice when the Navy required highly specialized timing devices for special operations.

Contract and Development

In the 1960s, the U.S. Navy awarded Hamilton a classified contract to develop a series of covert firing devices:

  • Development Site: China Lake Naval Weapons Center
  • Purpose: “To provide stand-off capability in the firing of explosive charges”
  • Concept: Disguise timing/firing mechanism as ordinary wristwatch for covert operations
  • Classification: Top secret during development and initial deployment

Operational Concept

The 308 series embodied a revolutionary approach to covert demolition:

  • Concealment: Device could be worn openly or carried without arousing suspicion
  • Versatility: Could initiate charges concealed in everyday objects—binoculars, canteens, flashlights, cameras
  • Stand-off: Allowed operators to set charges and evacuate before detonation
  • Plausible Deniability: Destroyed device left minimal evidence of sophisticated timing mechanism

Vietnam War Employment

The 308 series was used extensively by Navy SEALs during the Vietnam War:

  • Primary Users: SEAL Teams, Underwater Demolition Teams (including UDT-11)
  • Operations: Sabotage missions, harbor attacks, infrastructure demolition
  • Notable Missions: Employed in various classified operations throughout Southeast Asia

Operation Thunderhead (June 1972)

The 308 Bomb Timer is historically associated with this operation:

  • Mission: POW rescue attempt from North Vietnam
  • Forces: Navy SEALs and UDT-11 operators
  • Deployment: USS Grayback (submarine) and SEAL Delivery Vehicles
  • Outcome: Mission complications resulted in casualties; demonstrated dangerous nature of special operations

Post-Vietnam

  • Continued development of improved firing devices
  • Electronic timers gradually replaced mechanical watch-based devices
  • 308 series remained classified for many years
  • Documents declassified allowed historical acknowledgment

Current Status

  • Obsolete: Replaced by more advanced electronic firing systems
  • Historical Significance: Important milestone in special operations equipment
  • Collector Items: Rare surviving examples highly valued (most were destroyed in operational use)
  • Museum Pieces: Hamilton Watch Company museum preserves examples and documentation

Technical Specifications

SpecificationDetail
DesignationModel 308-6 (308 Series Remote Firing Device)
ManufacturerHamilton Watch Company
TypeElectrical time-delay firing device
DisguiseMilitary wristwatch appearance
Dial Format24-hour
Maximum Delay~24 hours
Output TypeElectrical (contact closure)
Firing ContactsTwo contacts on case back beneath plastic cover
Power SourceInternal (watch mainspring; possible battery assist)
Water ResistanceDesigned for field/maritime use
Era1960s development; Vietnam War deployment
ClassificationOriginally Top Secret

Frequently Asked Questions

Q: Why was a wristwatch chosen as the disguise for this firing device? A: A wristwatch provided ideal concealment—military personnel universally wore watches, so the device attracted no attention. The watch could be worn openly through checkpoints, stored without suspicion, and left at a target site without immediate recognition as an explosive component. Additionally, quality watch movements already provided the precision timing required for demolition operations.

Q: How does the Model 308-6 compare to standard Navy clockwork firing devices like the MK 12/13/15 series? A: The standard MK series devices were purpose-built demolition equipment, clearly identifiable as military ordnance. The 308 series was specifically designed for covert operations where recognition as a firing device would compromise the mission. While MK series devices offered various time ranges, the 308’s advantage was concealment rather than extended timing capability.

Q: What could the 308 series initiate? A: According to declassified documents, the 308 could initiate explosive charges concealed in everyday objects—binoculars, canteens, flashlights, and cameras were specifically mentioned. This allowed creation of sophisticated booby traps or time-delayed sabotage devices that appeared completely innocuous until detonation.

Q: How did operators avoid self-inflicted casualties with such a covert device? A: The 24-hour maximum delay provided substantial evacuation time. Operators would set the device, emplace the charge, and withdraw from the target area well before the timer completed. Strict operational procedures and timing discipline were essential. The covert nature also meant missions were carefully planned with precise timing calculations.

Q: Why are original 308 Bomb Timers so rare today? A: Most devices were destroyed in operational use—that was their intended purpose. Additionally, their classified nature meant strict accounting and destruction of unused devices. Surviving examples likely came from testing, training, or museum preservation. As a result, authentic original devices are extremely rare and valuable to collectors.

Q: Were there different models in the 308 series, and how did they differ? A: Yes, the series included multiple models (308-1 through 308-6 and possibly others). While specific differences are not fully documented in available declassified sources, variations likely included different time ranges, output specifications, waterproofing levels, or case styles optimized for specific operational requirements.

Q: How can a 308 Bomb Timer be distinguished from an ordinary Hamilton military watch? A: The critical distinguishing feature is the case back—authentic 308 devices have a plastic cover concealing two electrical firing contacts. An ordinary watch would have a standard metal case back without electrical contacts. However, positive identification should only be attempted by qualified personnel, as the concealment was intentionally effective.

Q: What was Hamilton Watch Company’s relationship with the U.S. military? A: Hamilton had supplied military timepieces since World War I and dramatically expanded production during WWII, providing over a million watches and thousands of marine chronometers. The company earned five Army-Navy “E” Awards for manufacturing excellence. This established relationship and proven reliability made Hamilton the logical choice for the sensitive 308 development contract.

Important Safety Notice

⚠️ All ordnance and firing devices, including those disguised as everyday objects, should be considered extremely dangerous until verified inert by qualified Explosive Ordnance Disposal (EOD) personnel. The 308 series was specifically designed to avoid recognition—never assume a vintage military watch is harmless. If you suspect a device may be a firing mechanism, do not handle it. Contact appropriate military or law enforcement authorities immediately. This information is provided for educational and historical training purposes only.

References: Declassified U.S. Navy contract documents; Hamilton Watch Company museum archives; Naval Special Warfare historical records; Vietnam War operational histories