The addition of a smooth radius to the mouth of each shell holder around the Shell Plate ensures smooth functioning--especially when the Shel Plate is used with the Lock N" Load Case Feeder. In addition, the bevel on the retainer spring groove helps keep the spring in place when inserting cases into the Shell Plate. Shell Plates for Lock N" Load AP and Pro-Jector are available to fit the calibers listed, but there are other calibers you can load with our shell plates. They will also hold additional calibers with the same case head dimensions.