F. Guffey comments:
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then there are ‘bumpers’ they bump the shoulder back .002 thousands, with out mentioning one word on how they accomplish such a feat
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Here's how its usually done. I and others have been doing this for almost 50 years; it's nothing new. Back then, we had to make our own tools as nothing long ago was neat as what's available now.
First, the fired bottleneck case has its case measured for case headspace from head to shoulder reference with a tool such as the RCBS Precision MIC. It's thimble's removed, a fired case is inserted base down then the thimble's screwed on gently until it's gaging surface stops against the fired case shoulder's reference point. The thimble's read off the index line same way as a micrometer is as it's graduated the same way. That number represents what the fired case headspace is.
Second, the fired case is full length sized in a die for it then measured again the same way. The reading after sizing is compared to the case reading with the tool before it was full length sized. It shows if the angled case between the neck and body (hereafter called the shoulder) has moved relative to the case head.
Third, if the reloader intended to "bump" the shoulder on the fired case back 2 thousandths after full length sizing it, then one of three things now exist. Two of those things require readjusting the die height in the press.
If the sized case headspace is now 2 thousandths shorter than before, it's been "bumped" back the desired amount.
If the sized case headspace is now more than 2 thousandths shorter than before, the die has to be readusted up a tiny bit.
If the sized case headspace is now less than 2 thousandths shorter than before, the die has to be readusted down a tiny bit.
Loosening the die's lock ring then rotating it about 1/10th inch in circumference on the die will relocate it about 1 thousandth of an inch. Should the die need to be moved down 5 thousandths so it will bump the fired case shoulder 2 thousandths back, then turn the lock ring about 1/2 inch in circumference on the die such that it moves higher up the die. Then lock the ring in place and screw the die down into the press.
Compare the above to thse instructions from RCBS for setting up a full length sizing die to bump the fired case shoulder back 2 thousandths.
http://www.rcbs.com/downloads/instru...ecisionMic.pdf