Hacker News Reader

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.

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

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

The more important point, I think, is that it means that the trebuchet is lethal with similar range to a handgun.

another way for america to avoid the metric system at all costs

No it's an old unit that's stuck around in the firearm world in basically every country. EU bullet makers specify their bullet weights in grains too.