The system dates back to medieval times when seed grains were the most accurate way to measure small amounts. These were used for precious metals, gemstones, and gunpowder. Originally, the seed was taken from the middle of an ear of barley, a seed unaffected by moisture and therefore more accurate. Using grains as a weight system allows for fine margins. One grain is just 0.002 of an ounce. So a 100 grain broadhead weighs 0.228 ounces. A 125 grain broadhead weighs 0.285 ounces. In most fine measuring, grams have taken over as the de facto standard, but in armory, grains are still common.