M42 or M-42 may refer to: In science: Messier 42, a nebula also called the Orion Nebula the 42nd Mersenne prime, 225964951-1, discovered in 2005 an Y-chromosomal mutation, see Haplogroup B-M42 In transportation: M42 motorway, a motorway in the United Kingdom M-42 (Michigan highway), a state highway in Michigan M42 (Cape Town), a Metropolitan Route in Cape Town, South Africa M42 (Pretoria), a Metropolitan Route in Pretoria, South Africa M42 (New York City bus), a New York City Bus route in Manhattan BMW M42, a 1989 automobile piston engine Dodge M42, ¾-ton command truck, 1950s M42 (sub-basement), a power station at New York City's Grand Central Terminal In photography: M42 lens mount, a standard for cameras In firearms and military equipment: Ag m/42, a Swedish semi-automatic rifle which saw limited use by the Swedish Army from 1942 until the 1960s M42 Duster, a United States Army self-propelled anti-aircraft gun M42 (gas mask), a United States military gas mask M/42 (bicycle), a Swedish military bicycle 45 mm anti-tank gun M1942 (M-42), a Soviet anti-tank gun (Motovilikha Plants) Duperite M42 helmet, an Australian WWII helmet M42 Stahlhelm, a type of German WWII helmet M42, a Smith & Wesson hammerless revolver M42 United Defense submachine gun, an American weapon used in World War II In manufacturing: M42 is the Unified numbering system code ("miscellaneous nonferrous metals and alloys") for a grade of high-speed steel with cobalt See also M1942 (disambiguation)