Making bullets go faster increases the kinetic energy even more. This effect is big, so people have tried (and succeeded) to make bullets that go faster and weigh less. Modern firearms take much lighter bullets and shoot them much faster than was the case in World War I. This makes the firearms better in two ways. The magazine (which holds the cartridges in the gun before they are fired) holds more of the smaller, lighter bullets. Also, the higher speed causes much more damage than the old-style bullets.