Scott,
Grain weight is a common measurement used in ammo manufacturing. 1 grain is 1/7000th of a pound. It is derived from a single grain of barley. It's part of the English weights & measures system. It has carried over into archery for arrow head weight. Bullets are designated in grains, as well as powder measures. Arrow heads usually vary in weight from 70-180 grains, with 125 grains as a "standard."
You can purchase a decent small grain/gram/oz scale for cheap. I bought one on ebay for about $30, from a reloading supplier.