Amuzzle blast is an explosiveshockwave created at themuzzle of afirearm duringshooting. Before aprojectile leaves thegun barrel, itobturates thebore and "plugs up" the pressurized gaseousproducts of thepropellant combustion behind it, essentially containing the gases within aclosed system as a neutral element in the overall momentum of the system's physics. However, when the projectile exits the barrel, this functionalseal is removed and the highly energetic bore gases are suddenly free to exit themuzzle and rapidly expand in the form of asupersonicshockwave (which can often be fast enough to momentarily overtake the projectile andaffect its flight dynamics), thus creating the muzzle blast.
The muzzle blast is often broken down into two components: an auditory component[1] and a non-auditory component.[2] The auditory component is the loud "Bang!" sound of thegunshot, and is important because it can cause significanthearing loss to surrounding personnel and also give away the gun's position. The non-auditory component is theinfrasoniccompression wave, and can cause concussive damage to nearby items.
In addition to the blast itself, some of the gases' energy is also released aslight energy, known as amuzzle flash.
Theaudiblesound of a gun discharging, also known as themuzzle report orgunfire, may have two sources: the muzzle blast itself, which manifests as a loud and brief "pop" or "bang", and anysonic boom produced by atransonic orsupersonic projectile, which manifest as a sharpwhip-like crack that persists a bit longer. The muzzle blast is by far the main component of a gunfire, due to theintensity ofsound energy released and the proximity to the shooter and bystanders. Muzzle blasts can easily exceedsound pressure levels of 140decibels, which canrupture eardrums and cause permanentsensorineural hearing loss even with brief and infrequent exposure.[3] With large guns with much highermuzzle energy, for instanceartillery, that danger can extend outwards a significant distance from the muzzle,[4] which mandates wearing ofhearing protections for all personnel in proximity foroccupational health purposes.
Forsmall arms,suppressors help to reduce the muzzle report of firearms by providing a larger area for the propellant gas to expand, decelerate and cool before releasing sound energy into the surrounding.[5] Other muzzle devices such asblast shields can also protect hearing by deflecting the pressure wave forward and away from the shooter and bystanders. Recoil-reducing devices such asmuzzle brakes however worsen potential hearing damage, as these modulate the muzzle blast by increasing thelateral vectors nearer to the shooter.
Theoverpressure wave from a firearm's muzzle blast areinfrasonic and thus inaudible to human ears, but it still can be highly energy-intense due to the gases expanding at an extremely high velocity. Residual pressures at the muzzle can be a significant fraction of the peak bore pressure, especially when short barrels are used. This energy can also be regulated by amuzzle brake to reduce the recoil of the firearm, or harnessed by amuzzle booster to provide energy to cycle theaction ofself-loading firearms.[6]
The force of the muzzle blast can cause shock damage to nearby items around the muzzle, and withartillery, the energy is sufficiently large to cause significant damage to surrounding structures and vehicles.[7] It is thus important for the gun crew and any nearby friendly troops to stay clear of the potential directions of blast vectors, in order to avoid unnecessarycollateral damages.
Typically the majority of the blastimpulse isvectored to a forward direction, creating ajet propulsion effect that exerts force back upon the barrel, resulting in an additional rearward momentum on top of thereactional momentum generated by the projectilebefore it exits the gun. The overall recoil applied to the firearm is thus equal and opposite to the total forward momentum of not only the projectile, but also the ejected gas. Likewise, the recoil energy given to the firearm is affected by the ejected gas. Byconservation of mass, the mass of the gasejectae will be equal to the original mass of the propellant (assuming complete burning). As a rough approximation, the ejected gas can be considered to have an effective exit velocity of where is the muzzle velocity of the projectile and is approximately constant. The total momentum of the propellant and projectile will then be:
where: is the mass of the propellant charge, equal to the mass of the ejected gas.
This expression should be substituted into the expression for projectile momentum in order to obtain a more accurate description of the recoil process. The effective velocity may be used in the energy equation as well, but since the value of α used is generally specified for the momentum equation, the energy values obtained may be less accurate. The value of the constant α is generally taken to lie between 1.25 and 1.75. It is mostly dependent upon the type of propellant used, but may depend slightly on other things such as the ratio of the length of the barrel to its radius.
Muzzle devices can reduce the recoil impulse by altering the pattern of gas expansion. For instance,muzzle brakes primarily works by diverting some of the gas ejecta towards the sides, increasing the lateral blast intensity (hence louder and more concussive to the sides) but reducing the thrust from the forward-projection (thus less recoil), with some designs claiming up to 40-60% reduction in perceived recoil. Similarly,recoil compensators divert the gas ejecta mostly upwards to counteract themuzzle rise. However,suppressors work on a different principle, not by vectoring the gas expansion laterally but instead by modulating the forward speed of the gas expansion. By using internalbaffles, the gas is made to travel through a convoluted path before eventually released outside at the front of the suppressor, thus dissipating its energy over a larger area and a longer time. This reduces both the intensity of the blast (thus lowerloudness) and the recoil generated (as for the sameimpulse,force isinversely proportional to time).
Muzzle blasts can stir up significantdust clouds, especially from large-caliber guns when firing low and flat, which can be visible from distance and thus give away the gun's position, increasing the risk of invitingcounter-fire. Preventive actions may consist of wetting the soil of the surrounding ground, having the muzzle brake vector to blast up and away from the ground, or covering the area around the muzzle with atarpaulin to shroud down as much airborne dust as possible.
Gunfire locators detect muzzle blast withmicrophones andtriangulate the location where the shots were fired. These are commercially available, and have been installed bylaw enforcement agencies asremote sensors in many high-crime rate areas ofurban centers. They can provide a fairly precise location of the source of a shot fired outdoors — 99% to within 33 feet (10 m) or better — and provide the data to policedispatchers within seconds of a firing.[8]