M1903 Springfield rifle
It was the US's main rifle during World War I. In 1936, it was replaced by the M1 Garand, a semi-automatic rifle. This was because semi-automatic rifles could fire bullets more quickly than bolt-action rifles. This was important because in World War I much of the fighting took place in trenches. In tight spaces like those in trench warfare, it is very important to be able to shoot quickly, and semi-automatic firearms do a better job at that than bolt-action rifles. However, it was still commonly used during World War II, and it became the main sniper rifle for the US. It was the main sniper rifle of the US until the early part of the Vietnam War.