In addition, heavier bullets carry more momentum than faster, lighter bullets. That difference in momentum grows as distance increases. The result is deeper penetration, and penetration and adequate bullet expansion are what kill game. A few years ago I fired a 250-grain bullet from a .35 Whelen at a bull elk. The bullet punched clear through the bull, which rolled over dead, even though the bullet packed only 1,800 foot-pounds of energy, about the same as a 100-grain bullet from a .243 Win.