The picatinny rail that I have used a single screw in the front to secure the barrel and the rail. While that method does work it does not allow the barrel/feed arm gap to be set properly. To get around this issue I counterbored the front hole to clear a socket and installed a taller set screw. This allows setting the barrel/feed arm gap, then securing the rail with a nut & washer. Since the rail was on the mill I relieved the rear to allow for easier handling of the pellet follower knob. Maybe this is how most rails are secured and I'm just late to the party.