Which of the following is not a PD fuze?

The M403 fuze is classified as an instantaneous impact fuze, which is designed for use in conjunction with certain projectiles but does not fall under the category of point-detonating (PD) fuzes. PD fuzes are designed to detonate upon impact with the target, ensuring immediate explosion upon contact. The M403, while a functional fuze in its own right, operates differently, focusing on detonation based on specific parameters rather than solely on impact.

In contrast, the M557, M739, and MK399 MOD 1 are all examples of PD fuzes. The M557 is widely used for artillery shells to initiate detonation upon striking a target, while the M739 is a PD fuze designed for use with certain types of munitions. The MK399 MOD 1 also serves as a PD fuze, designed to activate on contact with the target for effective operation. Each of these fuzes exemplifies the characteristics expected of a PD fuze, solidifying the M403 as the outlier in this context.