I believe the top rails are 4.0 mm wide from the outside of the receiver. The receiver sheet metal is 1.0 mm wide, so that would put the edge of the rail about tn the center of a 6.0 mm drill bit diameter. The top rails I believe are rough cut to 5.0 mm wide before stamping the step and final machining. I have references the making of the receivers in Russia including a general overview of the process but the description doesn't include all of the specs. 6.0 mm is the minimum rail gap, so somewhere between 6.0 mm and 6.3 mm should be ideal and may not have to be exact.