A muzzle blast is an explosive shockwave created at the muzzle of a firearm during shooting. Before a projectile leaves the gun barrel, it obturates the bore and "plugs up" the pressurized gaseous products of the propellant combustion behind it, essentially containing the gases within a closed system as a neutral element in the overall momentum of the system's physics. However, when the projectile exits the barrel, this functional seal is removed and the highly energetic bore gases are suddenly free to exit the muzzle and rapidly expand in the form of a supersonic shockwave (which can often be fast enough to momentarily overtake the projectile and affect its flight dynamics), thus creating the muzzle blast.

The muzzle blast is often broken down into two components: an auditory component and a non-auditory component. The auditory component is the loud "Bang!" sound of the gunshot, and is important because it can cause significant hearing loss to surrounding personnel and also give away the gun's position. The non-auditory component is the infrasonic compression wave, and can cause concussive damage to nearby items.

In addition to the blast itself, some of the gases' energy is also released as light energy, known as a muzzle flash.

The audible sound of a gun discharging, also known as the muzzle report or gunfire, may have two sources: the muzzle blast itself, which manifests as a loud and brief "pop" or "bang", and any sonic boom produced by a transonic or supersonic projectile, which manifest as a sharp whip-like crack that persists a bit longer. The muzzle blast is by far the main component of a gunfire, due to the intensity of sound energy released and the proximity to the shooter and bystanders. Muzzle blasts can easily exceed sound pressure levels of 140 decibels, which can rupture eardrums and cause permanent sensorineural hearing loss even with brief and infrequent exposure. With large guns with much higher muzzle energy, for instance artillery, that danger can extend outwards a significant distance from the muzzle, which mandates wearing of hearing protections for all personnel in proximity for occupational health purposes.

For small 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. Other muzzle devices such as blast shields can also protect hearing by deflecting the pressure wave forward and away from the shooter and bystanders. Recoil-reducing devices such as muzzle brakes however worsen potential hearing damage, as these modulate the muzzle blast by increasing the lateral vectors nearer to the shooter.

The overpressure wave from a firearm's muzzle blast are infrasonic 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 a muzzle brake to reduce the recoil of the firearm, or harnessed by a muzzle booster to provide energy to cycle the action of self-loading firearms.

The force of the muzzle blast can cause shock damage to nearby items around the muzzle, and with artillery, the energy is sufficiently large to cause significant damage to surrounding structures and vehicles. 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 unnecessary collateral damages.

Typically the majority of the blast impulse is vectored to a forward direction, creating a jet propulsion effect that exerts force back upon the barrel, resulting in an additional rearward momentum on top of the reactional momentum generated by the projectile before 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. By conservation of mass, the mass of the gas ejectae 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 ${\displaystyle \alpha V_{0}}$ where ${\displaystyle V_{0}}$ is the muzzle velocity of the projectile and ${\displaystyle \alpha }$ is approximately constant. The total momentum ${\displaystyle p_{e}}$ of the propellant and projectile will then be: