Re: Shims for dies to aid in headspace
Sully2, you said all the right stuff from the get go. You wanted to set fired case shoulders back more than the standard shell holder allowed.
Redding's competition shellholders start at the standard .125" height the go in .002" step above that. Each one reduces shoudler setback by its difference from standard; you didin't want those.
Most full length sizing dies are made to size fired bottleneck case shoulder back to about SAAMI specs. They do that when the die shell holder does that as it stops against the bottom of the die, then the case shoulder moves forward a bit. Virtually rimless bottleneck full length sizing dies with a chamber GO headspace in them and the shellholder on it will have a .005" gap between the die bottom and shellholder top. Just enough to let most cases end up at minimum SAAMI spec for case headspace.
Three things will set fired case shoulders back more. You did one of them. Another's to grind off a few thousandths from the top of the shellholder. The third one's go grind off a few thousandths from the bottom of the die.