Thanks for the Redding Die Instructions. I will give it a try.Well I see I am going to have to break this down Barney style and I am not the one showing his utter ignorance of precision reloading. You obviously do not understand tolerance stacking and SAAMI specs. You are going to have to put your thinking cap on here. It just so happened that Hornady has a nice video that you might want to watch and follow along to see how to actually do it correctly for precision.
BTW, 30 years LR competition experience, to include state champion, ranked nationally at 1k and have shot a 6" five shot group in mile match with 300 WSM and designed and loaded all kind of wildcats from custom 6 BR to 338/408. I understand precision reloading to be sure. I have been using custom shoulder stub guages made with my reamers for 30 years. And no must precision shooters are using custom/Whidden or Redding dies set up to allow precision bump with or without cam over.
To make it clear lets look at the 6.5 creedmoor to help understand the issue. SAAMI specs give a +- of .010 on the shoulder specs for this cartridge. Others can be as low as .007. In other words IF your chamber is absolutely within SAAMI dimension there can be a .010 shoulder variance from one gun to the other. Ammo has to minimal SAAMI at max dimensions so it will fit ANY gun. Add a worn die reamer and you could add another .002 variance. Did the mftr or gunsmith run the chambering reamer in .002 too long? That is why basic die instructions are written for the novice and are set up for cam over to get back to factory SAAMI specs. Their lawyers make them set the instructions up that way.
Now let's say you take a sizing die and set it down on the shellholder (again you must assume that they are all the correct height but I have seen .008 variance). I have an RCBS rockchucker and I do not care if you are Magilla Gorilla once metal meets metal that is all the compression and shoulder bump you are going to get. Period!!!!! Now IF you are lucky and IF you can actually measure shoulder bump you might be within .002 of the chamber shoulder. You could also in actuality be about .008 or more off. Again, think about it, that is why Redding has the competition shellholders that offer .0010 of adjustments in .002 increments for the guys hung up on shoulder bump or has dies that are a shade too long for correct bump. Keep reloading that same case time after time at that distance and you will eventually get case head separation. Think about this, how is it even possible to get case head separation IF cam over works perfect every time?
There is a reason the top two factory die mftrs (Whidden and Redding) give specific instructions for adjusting the dies for optimal shoulder bump and max case life to back off 1/3-1/2 turn and work your way down. On most dies 1/8 turn is about .009 of actual movement on the shoulder. As for Hornady dies, you might not know it but they actually have competition shooting teams that compete long range and do well with their dies. So not crap dies.
Just to put it to bed finally, we already had the Whidden instructions posted (BTW John Widden is world class champion and LR shooter so pretty sure he knows what he is talking about) and here is a composite of the Redding die instructions.
Cam over is for returning back to factory specs and the novice. Period