Does anyone know what size this is? Perhaps I can go buy one locally without having to contact Dillon.
The threads measure 28 per inch so it is a quarter inch fine tread allen screw. The trick will be finding the tip. My old 550 has the brass tip but they may have discontinued use of that. A call to Dillon would give you that info. Why would you remove the screw in the first place? In it's normal function you just loosen it a couple of turns and remove the shellplate bolt. The purpose of the brass tip would be to avoid damage to the shellplate screw. I would think the shellplate screw would be hardened and it would take more torque on the setscrew than needed to damage it. If it got scratched it could be dressed with a file easily in that non critical area so a plain hardware store setscrew might be fine for that purpose.