I apologize to you, F. Guffey, but when I refer to bumping the shoulder back, all I am trying to say is that I am bumping the shoulder down slightly until the bolt can close over it with hardly any resistance. I do this only on fired once or twice cases. In other words, I am sizing the cases to fill the camber the case has been fired from.
Thank you;
I find it impossible to bump/move the shoulder back as in shortening the case from the shoulder to the case head. I have suggested reloaders scribe the case body/shoulder juncture first before firing and or sizing. If it was possible to move the shoulder back the scribed line would move with the shoulder. And if there was any truth to the superstition that claims the firing pin drives the case forward until the case shoulder collides with the shoulder of the chamber; the scribed line would help the reloader understand what happens to the case when the trigger was pulled.
And then there were experiments, one armorer was convinced the case would separate with .060" clearance, he did not say if the head space of the case was .060" shorter than the head space of the chamber the case will suffer case head separation. He increased the length of the chamber, then chambered minimum length/full length sized cases in his 'wildcat' type chamber. The cases when fired did not suffer case head separation, I thought someone handed him the wrong rifle, careful reading of the test shows he was not using the rifle most thought he was using.
Had he scribed the case he would have know the shoulder on his fired cases did not move and the shoulder that formed when fired was a new shoulder that was 'about' .060" ahead of his scribed line.
And then there were experiments, one armorer was convinced the case would separate with .060" clearance, he did not say if the head space of the case was .060"
That was before the Internet, case head space had not been invented yet.
Again, I have chambered 8mm57 ammo in an 8mm/06 chamber. That is the equivalent to lengthening the 8mm57 chamber .227" (back to the top), if the armorer was convinced the case would suffer case head separation with .060" I should have been guaranteed a sure case of case head separation. After firing the cases I ejected 8mm57 cases that looked like
30/06 cases with just a hint of a neck. AND
I increases the length of the case from the (new) shoulder to case head .227".
Had my cases been driven to the shoulder of the chamber and the case locked onto the chamber when fired the neck would not have shortened and I would have proven the case was driven forward by the firing pin.
In the old days I suggested the reloader was always leaving something out or they were forgetting something or they just simply did not know.
Instead? I proved the shoulder on my cases did not move; instead my shoulder became part of the case body.
F. Guffey