Muzzle of a SIG 550 rifle.
The muzzle of a firearm is the end of the barrel from which the projectile will exit.
Precise machining of the muzzle is crucial to accuracy, because it is the last point of contact between the barrel and the projectile. If gaps exist between the muzzle and the projectile, escaping propellant gases may spread unevenly and deflect the projectile from its intended path (see transitional ballistics).
In the case of rifled weapons, the contour of a muzzle is designed to keep the rifling safe from damage, so it is commonly recessed or protected by a convex "crown".
When firing a gun, a flash is often seen at the muzzle (known as a muzzle flash) and is produced by the hot gases escaping the barrel. The size of the flash depends on various factors such as barrel length, type and amount of powder, etc. Flash suppressors are attached to the muzzle of the weapon to diminish these effects.
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The original article can be found at Muzzle (firearms) and the edit history here.