what is grain a measurement of
The grain is a measure of weight. 1 grain is equal to 1/7000th of a pound. Or 7000 grains are in 1 pound. There are 15.43 grains per gram for those who are metric. It's just a weird way of saying how heavy something is in firearms. More grains means heavier bullet.
The small golden disk close to the 5 cm marker is a piece of pure gold weighing one troy grain. Shown for comparison are a tape measure and coins of major world currencies. A grain is a unit of measurement of mass, and for the troy grain equal to exactly 7001647989100000000â 64.79891 milligrams. It is nominally based upon the mass of a single seed of a cereal.