The 6 mm SAW describes an experimental series of cartridge

Cartridge (firearms)

A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head or at its rim . Electrically...

s developed in the 1970s

1970s

File:1970s decade montage.png|From left, clockwise: US President Richard Nixon doing the V for Victory sign after his resignation from office after the Watergate scandal in 1974; Refugees aboard a US naval boat after the Fall of Saigon, leading to the end of the Vietnam War in 1975; The 1973 oil...

 for the U.S. Army

United States Army

The United States Army is the main branch of the United States Armed Forces responsible for land-based military operations. It is the largest and oldest established branch of the U.S. military, and is one of seven U.S. uniformed services...

. The cartridges were produced in a variety of sizes and from a variety of materials. The intent was to develop a cartridge that would replace all 5.56×45mm NATO weapons as well as most 7.62×51mm NATO

7.62×51mm NATO

The 7.62×51mm NATO is a rifle cartridge developed in the 1950s as a standard for small arms among NATO countries...

 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 machine gun

Machine gun

A machine gun is a fully automatic mounted or portable firearm, usually designed to fire rounds in quick succession from an ammunition belt or large-capacity magazine, typically at a rate of several hundred rounds per minute....

s in the U.S. military. Eventually, the 5.56 mm was retained and only the machine gun portion of the SAW (Squad Automatic Weapon) competition was successful. That program led to the M249 light machine gun

M249 light machine gun

The M249 light machine gun , previously designated the M249 Squad Automatic Weapon , and formally written as Light Machine Gun, 5.56 mm, M249, is an American version of the Belgian FN Minimi, a light machine gun manufactured by the Belgian company FN Herstal . The M249 is manufactured in the...

.

Note that the dimensions listed to the right are for 6×45mm SAW rounds. Three versions were produced for testing in the early 1970s. An aluminum-cased version was developed, as well as the 6 mm SAW Long which was slightly longer (by about 5 mm)