The M84 also known as a flashbang or stun grenade, is the currently-issued stun grenade of the United States Military and United States SWAT Military Police. Upon detonation, it emits an intensely loud "bang" of 170–180 decibels and a blinding flash of more than one million candela within five feet of initiation, sufficient to cause immediate flash blindness, deafness, tinnitus, and inner ear disturbance. Exposed personnel experience disorientation, confusion and loss of coordination and balance. While these effects are all intended to be temporary, there is risk of permanent injury. Consequently, the M84 is classified as a less-lethal weapon.
The M84 features a magnesium-based pyrotechnic charge inside a thin aluminum case, contained within a perforated cast steel body. Unlike the high explosives used in traditional ordnance, the pyrotechnic charge produces a subsonic deflagration, not a supersonic detonation, minimizing the blast effects. On initiation the auditory and visual elements of the deflagration being permitted to escape via the perforations in the cast outer body. This design minimizes the risk of collateral damage due to flame, blast and unconsumed fragments of the inner case.
It is intended to be thrown into enclosed spaces to distract and temporarily incapacitate enemy personnel for easier capture, or when risk of collateral damage during urban warfare or hostage rescue operations contravenes the employment of traditionally lethal and destructive fragmenting high explosive ordnance. US Army doctrine calls for the M84 to be deployed "during building and room clearing operations, when the presence of noncombatants is likely or expected and the assault element is attempting to achieve surprise."