A machine gun is a fully automatic firearm, most often separated from other classes of automatic weapons by the use of belt-fed ammunition (though some designs employ drum, pan or hopper magazines), generally in a rifle-inspired caliber ranging between 5.56×45mm NATO (.223 Remington) for a light machine gun to as large as .50 BMG or even larger for crewed or aircraft weapons. Although not widely fielded until World War I, early machine guns were being used by militaries in the second half of the 19th century. Notables in the U.S. arsenal during the 20th century included the M2 Browning .50 caliber heavy machine gun, M1919 Browning .30 caliber medium machine gun, and the M60 7.62×51mm NATO general-purpose machine gun which came into use around the Vietnam War. Machine guns of this type were originally defensive firearms crewed by at least two men, mainly because of the difficulties involved in moving and placing them, their ammunition, and their tripod. In contrast, modern light machine guns such as the FN Minimi are often wielded by a single infantryman. They provide a large ammunition capacity and a high rate of fire, and are typically used to give suppressing fire during infantry movement. Accuracy on machine guns varies based on a wide number of factors from design to manufacturing tolerances, most of which have been improved over time. Machine guns are often mounted on vehicles or helicopters and have been used since World War I as offensive firearms in fighter aircraft and tanks (e.g. for air combat or suppressing fire for ground troop support).