I have seen a YouTube video it's an adapter for break-action shotguns. If this would be added to the game i think about it in this concept: Adapter would take 1 inventory slot, rarely found in hunting cabins, deerstands, some village houses and other buildings that can spawn hunting equipment and weapons. It could be repaired it with gun cleaning kit. One adapter of course would take one barrel in a shotgun (So if you use a BK-43, you could make 1 barrel with adapter and 1 with 12cal). The adapter would not be capatible with BK-133. After gun was shot the case would stay in the adapter and our survivor would reload with special animation (It would take a little bit more time to reaload it, unlike regular 12 cal). I think this would be a nice help during the early game when you can't find any bullets. It would be cool if Bohemia would add it in the 1.26 update.
Ps. Sorry for all the gramatical, language, spelling or any other mistakes. As I don't know English very well :)