The most commonly used muzzleloader is the caplock. The caplock was invented in the early 19th century and uses a percussion cap to ignite the powder charge. Caplocks are much easier to use than flintlocks and were quickly adopted by militaries and hunters.
There are two main types of caplocks: side-lock and top-lock. Side-lock muzzleloaders have the lock (which contains the hammer, trigger, and other mechanisms) on the side of the gun, while top-locks have the lock on top of the barrel. Top-locks are more popular because they are less likely to jam than side-locks.
Muzzleloaders can be either single-shot or repeating rifles. Single-shot rifles must be reloaded after every shot, while repeating rifles have magazines that hold multiple rounds. Repeating rifles were not widely available until the late 19th century, so most muzzleloaders are single-shots.
Most modern muzzleloaders are inlines, meaning that the breech (the part of the barrel where you load powder and bullets) is in line with the barrel. Inlines are more accurate than traditional sideloading muzzleloaders because there is no offset between barrel and breech.