Grain is a measurement of weight for ammunition. One grain is equal to 64.79891 milligrams, and the grain of a bullet indicates the weight of the actual projectile fired. For example, a 200 grain .45 bullet weighs about 15 grams.
Grain is an important consideration when shopping for ammunition. Lighter bullets typically move faster and have the capacity to deliver more kinetic energy to the target. Heavier rounds offer more stopping power and can do more damage when striking tissue. However, weight alone is not the only factor that affects the damage potential of a bullet. The type of bullet also has much to do with its damage capability. Hollow point ammunition causes more tissue damage than full metal jacketed bullets, which remain intact as they pass through flesh.
Grains can also refer to the amount of gunpowder used in a bullet. In these terms, 7,000 grains equal 1 pound of gunpowder, and 437.5 grains equal 1 ounce of gunpowder.
The grain measurement dates back to ancient times. While it was always meant to correspond to the weight of a single grain of cereal, the measurement was largely a symbolic one, and transactions did not use actual grains of wheat.