This term denotes any gun that is loaded from the front, or muzzle. Traditionally these are very old guns, most often rifles but also including shotguns and pistols, but often they are guns of much more recent manufacture, used by hunters.
These guns are loaded by first clearing the bore of oil and other substances, then loading a measured powder charge, followed by the projectile(s). In the case of a shotgun, multiple projectiles will be used.
Older styles look like the old rifles you may have seen in the movies, with the hammer off to one side of the rear end of the barrel, usuallly the right. This is known as a sidelock design.
In recent years the trend has been towards "in-line" designs, which place the components of the gun and load all in a row, i.e. in-line with one another. Generally these designs offer better reliability and may fend off foul weather more efficiently than sidelock guns.
All varieties of these guns can be extremely accurate (despite campfire talk to the contrary), but they suffer the disadvantage of being quite slow to load, so you generally only get one shot at the game you're pursuing.
Boy Scout and Varsity Scout Muzzle loading Firearms
Because of the historical significance of muzzle loading firearms, Scouts are encouraged to learn to safely load and shoot a muzzle loader. On the range, each Scout must be under the direct supervision (one on one) of a currently NRA or NMLRA certified muzzle loading instructor when loading and firing the muzzle loader.
Muzzle loading rifle. Only recently manufactured (or assembled from a kit) percussion cap muzzle loading rifles no greater than .58-caliber are to be used. Flintlock rifles are not approved for use by Boy Scouts or Varsity Scouts. All muzzle loading rifles are subject to safety inspection by the instructor or range officer. Rifles made from kits must be checked by a qualified gunsmith.
Propellant. Only a commercially manufactured, sporting grade black powder or black powder substitute offered for sale by a reputable firm should be used in muzzleloading rifles.
Muzzle loading pistol. (No handguns are permitted in the Boy Scout and Varsity Scout programs.)
Muzzle loading shotgun. Only recently manufactured (or assembled from a kit) percussion cap muzzle loading shotguns no smaller than 20-gauge or greater than 10-gauge are to be used. Flintlock shotguns are not approved.
Propellant. Only a commercially manufactured, sporting grade black powder or black powder substitute offered for sale by a reputable firm should be used in muzzle loading shotguns. For new shooters, the amount of propellant in grains should be at the minimum of the gun manufacturer’s recommended load range.