Muzzles are extremely important to realizing the protential accuracy of a firearm, as they are the last points of contact the barrel has with the bullet. If they are not concentric, the gases, escaping when the bullet leaves the barrel, may spread unevenly, potentially deflecting the bullet from its path.
The type of contour on a muzzle is typically designed to keep the rifling in a barrel safe from damage, so it is commonly recessed or protected by a convex "crown."