Bullet weight is stated in grains, a unit inherited from apothecary weights. There are 7,000 grains in a pound, so a 55-grain .223 bullet weighs about an eighth of an ounce. The number matters because it sets how fast a load can go, how well it fights the wind and whether your barrel can stabilize it. Light bullets: speed and a flat path In the small-bore family, 50 to 55 grains is the...