there are no mechanical safety devices. The characteristics of the fuze compound and its metal exterior therefore have to be extremely carefully judged to ensure the necessary combination of reliable detonation with safety when mishandled. Put simply, the shell must never detonate when dropped point-down onto a concrete floor, but must always detonate when it hits a target at high velocity. Raufoss multipurpose ammunition uses such fuzes.

Fuzes which are detonated by the close proximity of the target are unsurprisingly known as proximity fuzes, although for security reasons during the Second World War (when they were first introduced) some disinformation was circulated to the effect that their USN code letters - VT - stood for variable time. Initial theoretical work was carried out in the UK in 1939^40, but the concept was transferred to the USA for development and production. These immediately achieved an improvement in AA successes of between three and five times in comparison with timed or contact fuzes. There are various methods of achieving detection but a small radar set is by far the most common in cannon shells, which limits its application to the larger calibres (originally 75mm but now down to about 40mm), partly because of the size of the fuze and partly because smaller shells do not contain enough HE to damage the target at a distance.

Because of the difficulty of detecting other targets against background clutter, proximity fuzes for automatic cannon are used almost exclusively against aircraft or missiles. They can normally be set to detonate at different distances according to the nature of the target or of the environment, with a smaller distance being set when firing at targets close to the ground or sea to ensure that the fuze is not triggered accidentally. In recent years more sophisticated fuzes have been developed to deal with this problem.

Modern electronic fuzes can be extremely versatile, such as the Bofors programmable fuze which forms an essential part of the 3P ammunition already mentioned. As well as the normal proximity function, the fuze can operate in five other modes: gated proximity (fuze only activated close to the target to avoid premature detonation); gated proximity with impact priority (a slight delay in activation to provide the opportunity for a direct hit); time function (for airburst fire to provide a shrapnel effect against surface targets); and two impact functions with variable delay.

The vast majority of fuzes are fitted to the points of projectiles. However, some types of shell make use of base fuzes, as in the case of the SAPHE shells already mentioned. Some fuzes are associated with particular types of shell. For example, mine shells produce few fragments, relying almost entirely on blast and incendiary effects, so need to explode inside the target. They are therefore used with delayed-action contact fuzes. Shells exploded outside the target by proximity fuzes cause most damage by means of fragments and therefore either contain separate pellets (as in the Bofors PFHE) or have a thick case which is designed to break up into fragments of a specified range of sizes.

There is the obvious risk that shells fired at an attacking aircraft might carry on to detonate on landing some distance away, possibly among friends. Contact and proximity-fuzed shells are therefore often fitted with a self-destruct device which detonates the shell after a certain period of time, when it can be presumed to have missed the target. Sometimes this is a function of the fuze, but a simpler system used in smaller calibres is for the propellant to ignite a separate chemical fuze in the base of the shell, a reversion to the original form of time fuze. This may also be achieved by the final stage of a tracer burn.

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