What makes a firearm a "musket" is the lack of "rifling" (spiral grooves cut into the inside of the barrel to make the bullet spin on exiting the barrel for greater distance and accuracy.)
Muskets were smooth-bore--no rifling. They weren't nearly as accurate, nor would they shoot as far as a rifle, but muskets continued to be the preferred weapon for infantry troops until the late 18th century, because they were faster and easier to load than a muzzle-loading rifle, because the load and wad had to be pushed down the barrel past the rifling grooves, which slowed the process down.
There is a little more info on this subject in an article I wrote on my website. See
Revolutionary and Civil War Muskets
Hope this helps