A ball sizer for a paint-ball gun includes a bore having an entrance. In the preferred embodiment, the bore has a chamfer, which in turn has a smooth transition from the entrance to the bore of the sizer. The transition may define a section of a parabola, a section of a circle, or a combination of a straight line with a transition to a curve, such as a section of a parabola or circle. Similar transitions may be provided at the exit of the ball sizer, and at the entrance to the barrel attached to the exit of the sizer, or both.