Fuze M78

Impact fuze with inertial arming system M78 (M78A1)
are used with concrete-piercing projectiles of calibers from 75 mm to 280 mm, except 175 mm.

The M78 (M78A1) fuze consists of two parts – the fuze itself and the M25 booster, which are separate parts but are supplied and packed together. The M25 was designed specifically for the M78 series fuzes and cannot be used with other fuzes.

M78 (M78A1) fuzes come in two types: without delay (for use against targets located on the surface) and with a 0.025-second delay (for use against concrete targets).

M78 (M78A1) instantaneous impact fuzes without delay have a nose section painted white, about 2.5 cm wide from the tip.
The fuze has a strong hardened-steel body with an oval nose. The fuze body contains a striker and an inertia-type delay plunger mechanism with a detonator. The delay plunger is locked by two spring fingers.

Functioning:

After firing, the force of inertia releases the pin from the lock. As the projectile's rotation speed increases, centrifugal force pulls out the two piston pins of the delay plunger in the detonator head, thereby arming the delay plunger.
At the same time, centrifugal force withdraws the locking pin, allowing the rotor (slider) to turn on the pivot pin until the detonator is aligned with the ignition hole in the upper charge cover.
The rotor (slider) is locked in the active position by a centrifugal lock during the rest of the flight. On impact, the delay plunger moves forward by inertia to the firing pin and initiates the explosion.