The numbers on a box of bullets or powder indicate bullet weight but not the load. Bullet weights can range from 35 grains for heavy target loads to 60+ Grains for centerfire rifles. However, 5 grains are considered standard across all powders (there are some exceptions). Therefore, the grain unit system they can measure stands at 1/8th oz avoirdupois - meaning that it's equal to Movie Ounce .44 Mag/.