Grains Apothecary is used to measure powder charge weight is because it was a “fine” enough scale for measuring small amounts of things that if you get it even a tiny bit wrong, can kill you. So, ammunition manufacturer’s looked around and scales used for accurately measuring small amounts of drugs were commonly available, so they went with that.
Cool side point: Powder charges are checked by weight and dosed out, (or thrown), by volume as it has always been done since the first gonnes were a thing.
Grains Apothecary is used to measure powder charge weight is because it was a “fine” enough scale for measuring small amounts of things that if you get it even a tiny bit wrong, can kill you. So, ammunition manufacturer’s looked around and scales used for accurately measuring small amounts of drugs were commonly available, so they went with that.
Cool side point: Powder charges are checked by weight and dosed out, (or thrown), by volume as it has always been done since the first gonnes were a thing.