Which of the following items is prohibited from being stored in a small arms locker?

The prohibition of 20mm CIWS rounds from being stored in a small arms locker is based on the significant differences in the nature, size, and purpose of such ammunition compared to the other options listed. Small arms lockers are specifically designed for the secure storage of conventional small arms ammunition, which typically includes items such as handgun ammunition, rifle ammunition, and shotgun shells. These types of ammunition are generally compatible with the firearms typically stored alongside them in these lockers.

In contrast, 20mm CIWS (Close-In Weapon System) rounds are much larger and intended for use in specialized military weapon systems designed for defense against aerial threats, such as missiles and aircraft. These rounds require specific handling, storage facilities, and logistics due to their size and the high energy they contain. Therefore, storing them in a small arms locker would not only be impractical but also against safety protocols and regulations regarding ammunition storage. The distinction between the types of ammunition highlights the importance of appropriate storage facilities for different classifications of ordnance.