A muzzleloader is any firearm into which the bullet is loaded from the muzzle of the gun. This can apply to anything from cannons to pistols, but in modern parlance the term most commonly applies to black powder small arms.

In general, the sequence of loading is to put in: a) gunpowder, either by inserting pre-measured pellets, pouring in loose powder, or by inserting a pre-measured bag of gunpowder usually called a "cartridge", b) wadding, soft material like cloth or paper, and c) the projectile, which may range from a solid lead ball to jacketed sabots. The gunpowder used is typically blackpowder or blackpowder substitutes like Pyrodex.

Since the items are generally tight-fitting, and the barrel long, a tool called a "ramrod" is used to push the items down.

Muzzle-loaders must usually be cleaned ("swabbed") before reloading, so that there are no bits of burning material left in the barrel that may set off the next charge of gunpowder.

Modern muzzleloading firearms range from reproductions of sidelock , flintlock and percussion long guns, to in-line rifles that use modern inventions such as a closed breech, sealed primer and fast rifling to allow for considerable accuracy at long ranges. There is even a clever selective fire muzzleloader with electronic ignition in development, known as the Metal Storm.