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Ordnance, Explosives, and Related Items




The AN-M103 was the standard nose fuze in service. When shipped, the fuze is set for delay action. One auxiliary booster is required when this fuze is used in Navy bombs. When used in light-case, fragmentation, chemical, incendiary, and aircraft depth bombs, the fuze should be set for instantaneous action to prevent breaking up of the bomb case before detonation occurs.

The Nose Mechanical Impact Fuze M103 had 32 single threads per inch on the arming screw, instead of 28 double threads per inch on the AN-M103, resulting in an arming time of 850 vane revolutions for instantaneous firing and 525 vane revolutions when set for delay action. The M103 had larger and weaker vane construction.

Especially large vanes have been designed for the AN-M103 for use with flatnosed depth bombs, as the regular vanes have difficulty in arming.

The AN-M103 can be used for dive bombing, but not for masthead bombing.


When bomb is dropped, the arming wire is pulled and the vanes rotate. The two pinion gears are rotated counterclockwise, their off-center hubs walking the external-tooth gear backwards around the teeth of the internal- tooth gear, which is thus cranked in a counterclockwise direction. The outboard edge of the internal-tooth gear is grooved and rides screws projecting through from the vane cup, maintaining an even position. The arming screw, being positively attached to the base of the internal tooth gear, is gradually unthreaded from the striker. As it unthreads, it lifts the entire vane assembly, including the vane cup. After 220 revolutions of the vanes, the vane cup will clear the safety discs, which spring free, leaving the striker secured only by the shear wire and the setting pin. The spring-loaded arming stein will rise as the vane assembly rises, being retained only by the base of the internal-tooth gear. If the setting is for delay action, however, the setting pin will be depressed into the deep slot and will protrude into the channel of the arming stem to engage the collar on the arming stem after it has risen only sufficiently to clear the step in the detonator slider, lining the detonator up with the delay firing train.

On impact, the force of inertia will cause the striker body to shear the shear pin and setting pin. and the delay firing pin will impinge on the delay primer, setting off the flash which ignites the delay pellet, relay, primer, detonator, booster lead-in. and booster in succession. The instantaneous firing pin will merely protrude into the empty channel positioned to receive it. If the fuze is set for instantaneous action, the setting pin will be in the shallow slot and will not protrude into the arming-stem channel. After an additional 1L0 revolutions of the vanes (330 total), the vane cup will have been lifted high enough to have the arming stem moved out of the slider cavity, allowing the slider to align the primer detonator beneath the instantaneous firing pin. The slider is motivated by two springs and is locked in the armed position by a spring-loaded detent. On impact, the firing pin will impinge directly on the primer, setting off the detonator, booster lead-in, and booster in succession. Even though an instantaneous setting is used, the delay striker will impinge on the delay primer, setting off the delay pellet. It is thus possible that the fuze would function on delay, even though set for instantaneous action, if it failed to function instantaneously.

Hazardous Components

No information about hazardous components.

See Also

Nose, Impact, M148


TM 9-1385-51, Ammunition (Conventional) for Explosive Ordnance Disposal (1967)

TM 3-400, Chemical Bombs and Clusters (1957)

OP 1664, Volume 2 - US Explosive Ordnance (1947)

USNBD - Bombs and Fuzes, Pyrotechnics (1945)

TM 9-1904, Ammunition Inspection Guide (1944)