It’s a super old unit of measurement for mass [0], idea being that a “grain” is a single seed of wheat etc.

1 gram is a little over 15 grains, and there are 7000 grains per (avoirdupois) pound.

0: https://en.wikipedia.org/wiki/Grain_(unit)

325 mg aspirin pills were originally 5 grains. A carat was 4 grains. The unit was used by apothecaries, jewelers, and bullet makers.

It's a unit of mass used in the firearms world