Muzzle Ring Diameter is the distance between points A & B.

The inside Muzzle Ring Diameter is best measured with a good pair of calipers to the nearest tenth of a millimeter. This measurement should be taken from the front or blade side of the muzzle ring. Frequently the muzzle ring has been bent or distorted, so measurements from multiple angles should be taken to achieve an estimated average.

It is not unusual to find variances of a few tenths of a millimeter between bayonets of the same type.