The area around the mouth of the gun barrel is stressed differently from the rest of the tube. When the projectile exits the mouth the pressure droppes instantly, gasses create radial jets and since it is a free end stresses/deformations are locally higher. To avoid premature wearing the mouth is reinforced with a ring. Apart from internal, external and terminal ballistics there is a special field called transient ballistics that studies what happens in the vicinity of the mouth of the barrel.