Grain

Grain is a measurement of bullet weight. 437.5 grains equals one ounce. This is an important aspect of a bullet because it determines the overall mommentum and energy of the projectile. If a 150 grain (.342 ounce) .30 caliber slug is traveling at a velocity of 2900 ft/sec (1977 mph), then the bullet is carrying 2820 ft-lbf of energy. The same caliber slug traveling the same speed but weighing 180 grains (.411 ounces) carrys 3315 ft-lbf of energy. That is a difference of 495 ft-lbf.  Different weight can be obtained by a minor change in length or by manufacturing the bullet projectile of different materials.  

The grain of the bullet determines key aspects of a bullet's effectiveness.  A heavier grained bullet will penetrate deeper in a target because of its increased area.  A lighter bullet will travel at a higher velocity than a heavier bullet given the same powder load.