Originally Posted by Ludicrous
Roger that. I'm bumping shoulders back .002 and wasn't quite sure if it made any difference at all where the measurement is taken. Thanks.
I always CAD out a once fired case (actually five cases), and setup my own set of numbers. Try to split the difference between the neck O.D. and the shoulder diameter; give or take .010". This becomes my own gauge dine and I then calculate the distance from the face of the case to the gauge line. Thus creating the gauge line deminsion. I may adjust the length a couple thousandths as things begine to settle in, but rarely more than four thousandths, and usually less than two. I donot trust generic headspace gauges for this as chambers will vary slightly. You culd probably use something like a 12mm or even a 7/16" inner race from a needle bearing as long as it's longer than the neck. (you know right upfront that the faces on an inner race are parallel). Or simply make your own in a lathe.
You are correct in bumping the shoulder back .002" to .003", but that number becomes generic if you base everything off an existing drawing. I prefer to do this by chambering a case in the rifle looking for a slight bit of resistence while I close the bolt. Then I bump everything back from there. I also will vary the amount of bump per individual chambering. (I only bump .222's and .223's back about .001" max). I keep a log book on what I do here as I'm getting old now!
I might also add here (although slightly off topic) that one of the first things I do is to find out just how long the actual neck is in the chamber, and setup a max overall length from that distance; minus .015" (I hate trimming cases almost as much as I hate reaming primer pockets)