Magazines were proposed on two occasions by chaosvolt and I, but circumstances prevented their addition. I have some magazines that can be PRd.
Metal fragments lack range. I don’t think they are intended as BB ammo in the first place.
EDIT: Ah, they are supposed to work with BB after all. The problem is that BBs use ammo stats to set range and shards use the splinterguns stats to set range; this means that BBs can be used with splinterguns, but shards fail to fire from BB guns. I may fix this later.
If you want to fix it yourself, add "range": 12, to the BB gun and remove “range”: 12, from the BB.