A bayonet lug is a standard feature on most military musket

Musket

A musket is a muzzle-loaded, smooth bore long gun, fired from the shoulder. Muskets were designed for use by infantry. A soldier armed with a musket had the designation musketman or musketeer....

s, rifle

Rifle

A rifle is a firearm designed to be fired from the shoulder, with a barrel that has a helical groove or pattern of grooves cut into the barrel walls. The raised areas of the rifling are called "lands," which make contact with the projectile , imparting spin around an axis corresponding to the...

s, and shotgun

Shotgun

A shotgun is a firearm that is usually designed to be fired from the shoulder, which uses the energy of a fixed shell to fire a number of small spherical pellets called shot, or a solid projectile called a slug...

s, and on some civilian longarms

Long gun

The term long gun is used to describe classes of firearm and cannon with longer barrels than other classes. In small arms, a long gun is designed to be fired braced against the shoulder, in contrast to a handgun, while in artillery a long gun would be contrasted with a howitzer or carronade.-Small...

. It is intended for attaching a bayonet

Bayonet

A bayonet is a knife, dagger, sword, or spike-shaped weapon designed to fit in, on, over or underneath the muzzle of a rifle, musket or similar weapon, effectively turning the gun into a spear...

, which is typically a long spike or thrusting knife. The bayonet lug is the metal mount that either locks the bayonet onto the weapon or provides a base for the bayonet to rest against, so that when a bayonet thrust is made, the bayonet does not move or slip backwards. Less than 400 years ago, bayonet lugs or their predecessors that allowed them to slip over the barrel did not exist.

Prior to the lug's invention, bayonets merely stuffed into the muzzle's end from a tight-fitting stub, rendering the firearm virtually useless and certainly preventing it from being discharged. But by the late 17th century, this type of bayonet was entirely phased out and subsequently replaced with the socket bayonet, that slides over the muzzle with the blade offset to the side, just above or underneath. Bayonet lugs are usually located near the muzzle

Muzzle (firearm)

The muzzle of a firearm is the end of the barrel from which the projectile will exit.Precise machining of the muzzle is crucial to accuracy, because it is the last point of contact between the barrel and the projectile...

 end of a musket, rifle, or other longarm barrel

Gun barrel

A gun barrel is the tube, usually metal, through which a controlled explosion or rapid expansion of gases are released in order to propel a projectile out of the end at a high velocity....

. The lug is occasionally placed on top of the barrel, if serving as the base of the front sight

Iron sights

Iron sights are a system of shaped alignment markers used as a sighting device to assist in the aiming of a device such as a firearm, crossbow, or telescope, and exclude the use of optics as in telescopic sights or reflector sights...

, or more often mounted to the side or bottom of the barrel.