Knife bayonet

US Marines posing with knife bayonets attached to their rifles.

A knife bayonet is a knife which can be used both as a bayonet, fighting or utility knife. The knife bayonet became the almost universal form of bayonet in the 20th century due to its versatility and effectiveness. The socket bayonets proved useless when separate from the rifle and useless in trench warfare; and while versatile, sword bayonets proved to be impractical weapons in trench warfare because of their length.

The knife bayonets are basically fighting knives or utility knives with a lug and/or muzzle ring to attach to the barrel of a firearm such as an assault rifle or submachine gun. Almost all bayonets today are knife bayonets that are designed for use in hand-to-hand fighting and as utility knives. Some have serrated (saw-blade) back edges for utility and be usable as wire cutters (when used in conjunction with fittings on the sheath).

M9 Knife Bayonet.

Modern knife bayonets have 12 cm to 23 cm long and 2 cm to 5 cm wide blades, and are often fullered. They have been forged for durability so as not to break when twisting; this was often the problem with long and slender socket bayonets. Like sword bayonets, they can be used not only for thrusting, but also for cutting.