It’s a skull-mounted personal protection device. Originally capable of only of single-shot roundball projectiles, the devices remained unpopular until the introduction of bullet-based recoilless designs as the firing of the initial models, while effective, tended to leave the user with severe whiplash unless they were possessed of large neck muscles. Later innovations included the addition of a curved magazine that wrapped around the back of the head to provide multi-shot capabilities. More recent models have tended to use flechette ammunition due to weight concerns. The DoD is said to be researching laser- and/or maser-based devices. As power density is always a main concern for electrically-based weapons, they are reportedly focusing on subcritical nuclear thermal generation with a hybrid americium/plutonium core.