Original: In the 18th century, improvements in ammunition and firing methods allowed rifling to be practical for military use, and the term "rifled gun" gave way to "rifle".
Simplified: In the 1800's, better ammunition and ways of firing the gun, meant that rifling was practical for military use, and the rifled musket became common. A few years later, the use of cartridges, breechloading, and multiple rounds of ammunition meant the "rifled musket" gave way to "rifle", and "muskets" were no longer used.