The helmet may not be used for which of the following purposes?

The helmet is a critical piece of personal protective equipment designed primarily to safeguard the wearer's head from injuries sustained during combat or training environments. Its construction and materials are specifically engineered for impact protection, not for performing tasks that could damage the helmet or compromise its integrity.

Using the helmet as a hammer could lead to physical damage to the helmet, undermining its protective capabilities. The materials used in a helmet can crack or deform when subjected to blunt force impacts that are not representative of head trauma. Consequently, if the helmet is later needed for its intended purpose, it might not provide adequate protection due to the damage incurred while being misused. This emphasizes the importance of using each piece of gear strictly for its intended purpose, ensuring maximum safety for the individual in high-risk situations.

In terms of the other options, using a helmet to carry water, store ammunition, or protect against blinding light can either be safer or more acceptable than using it as a hammer. However, these uses are also limited by practical considerations and regulations, but they do not compromise the helmet's structural integrity in the same way as the first option does.