40mm

40 millimeters, abbreviated 40 mm, is a unit of length in the metric system equal to 0.04 meters (approximately 1.57 inches). The measurement is widely used to specify dimensions such as bore diameters and lens focal lengths, among other applications.

In firearms and ammunition, 40 mm most commonly refers to a caliber used by grenade launchers. The 40×46 mm cartridge is a standard low-velocity munition for under-barrel and standalone launchers such as the M203 and M320. 40 mm grenades come in a variety of payloads, including high-explosive, fragmentation, smoke, illumination, and non-lethal types, designed for indirect-fire or shorter-range engagements.

The term is also associated with the 40 mm Bofors autocannon, a widely produced anti-aircraft gun developed by the Swedish company Bofors. Entering service in the 1930s, it became a staple on ships and in land installations during and after World War II, valued for its relatively high rate of fire and effectiveness against aircraft at moderate altitudes. The Bofors platform also influenced later artillery and naval armaments that used the same caliber.

In photography and videography, 40 mm denotes a lens focal length. On full-frame cameras, a 40 mm lens offers a moderately wide field of view, while on smaller sensors the effective perspective changes with the crop factor. Across these contexts, 40 mm serves as a dimensional descriptor rather than a single technology or system.