December's Child is a unique combat rifle. It bears an equally unique M4 Carbine legendary modification that reduces the rifle's weight, converts its caliber to 5.56mm instead of the standard .45, and increases its firing rate by 25% and reload speed by 15%.
It is found inside the Vim! Pop factory, in an area that DiMA used as a secret medical lab. He had an AI named KYE 1.1 guard his facility, and if Nick Valentine is currently accompanying the Sole Survivor, KYE 1.1 will recognize him as a synthetic prototype and let them enter. A medium speech check can be used to gain entrance without Nick being present, though. Regardless of the method used, December's Child is sitting beside the medical equipment at the back of the room.
The value of the gun's special properties is dubious for a variety of reasons.
Despite claiming to have reduced weight, December's Child weighs exactly as much as a regular combat rifle.
December's Child boasts exactly the same damage rating as a regular combat rifle despite using a supposedly more powerful round. While .45 rounds are one of the more uncommon types of ammo themselves, 5.56mm rounds are even harder to come by.
The weapon can still be modified to fire .38 rounds or .308 rounds, thus making its most distinctive property entirely moot. Doing so basically turns December's Child into a regular combat rifle with the Rapid legendary prefix.
The legendary modification's name does not show up in-game. However, in the game files it is labeled as "M4 Carbine".
December's Child can be obtained before Best Left Forgotten is started.
December's Child is currently the only gun in the game other than the assault rifle that can fire 5.56mm ammo, however it has increased damage/bullet and lighter weight compared to non-Legendary assault rifles.
DiMA's AI KYE 1.1 will open the door only a single time before going into sleep mode for the rest of the game. The door will be locked again upon returning, but this does not make the gun unobtainable. There exists a documented method in which one can use various exploits to enter the room, obtain the weapon and escape the room without the need of KYE 1.1.