Shoulder back or Datum line back?

[BoydAllen]

I have assumed when we talk about bumping the "shoulder back", we are actually measuring the Datum line to the case base. But I got challenged by that this week.
I should have bet a steak dinner on that. So what are we referring to?

Not all comparitors measure off of the datum line. The Sinclair tool has inserts that are angled to match the case shoulder. Another tool is sometimes furnished by smiths along with a new barrel. The old school term is a "gizzy", It is a leftover piece of the barrel with the ends squiared off and the reamer run into one end just short of the base of the shoulder.

 

[gdbleb]

I'm heartened to hear other's comments about Guffey's write-ups. I could often keep up with the first half of a write-up but then he went into outer space on the second half of the write-up. Sometimes it was in the opposite order.
Re the datum line, you could make (or have made) a caliper tool insert with the exact ID specified in the SAMMI drawings. I tried this approach and was somewhat successful in getting close to spec measurements. You might find a neck bushing that would have the exact ID although that would be a large bushing. Anyway, many here have critiqued that approach as having issues with irregular seating of the insert on the shoulder and thus getting inconsistent measurements. I experienced this myself.
The alternative is to have a bevel on the insert that may or may not match the spec angle of the shoulder and then you get more consistent seating of the insert to the the shoulder but you are back to having a comparator.
I finally settled on an approach that uses "go" gauges and Whidden shoulder setback gauges. So I have a fixed reference and a comparator. Thank goodness I don't have too many calibers.

 

[BoydAllen]

I'm heartened to hear other's comments about Guffey's write-ups. I could often keep up with the first half of a write-up but then he went into outer space on the second half of the write-up. Sometimes it was in the opposite order.
Re the datum line, you could make (or have made) a caliper tool insert with the exact ID specified in the SAMMI drawings. I tried this approach and was somewhat successful in getting close to spec measurements. You might find a neck bushing that would have the exact ID although that would be a large bushing. Anyway, many here have critiqued that approach as having issues with irregular seating of the insert on the shoulder and thus getting inconsistent measurements. I experienced this myself.
The alternative is to have a bevel on the insert that may or may not match the spec angle of the shoulder and then you get more consistent seating of the insert to the the shoulder but you are back to having a comparator.
I finally settled on an approach that uses "go" gauges and Whidden shoulder setback gauges. So I have a fixed reference and a comparator. Thank goodness I don't have too many calibers.

Since your desired bump is from the actual maximum possible from a given barrel on a particular rifle. Go gauges are sort of irrelevant to the process. My old Stoney point set has radii where the inserts contact the case shoulder and I have found that a very good configuration for use as a comparitor. The only reason that we bump back shoulders it to create a little clearance in that direction on an as needed basis. Most of the time one firing on a new case will not require any shoulder bump. If you set your die to produce the exact shoulder to head dimension of that once fired case, given the reduction in diameter along the body, the case will usually chamber freely. Adding more bump would only cause additional case stretch.

 

[Fast14riot]

"Datum diameter" is a waste of time. We're measuring comparative measurements, from fired to sized. Just use the same tool every time. I literally have 9mm cases as comparators for some chamberings, the case lives in the die box so I use the same one every time and it matches my notes. Its when you start changing things you measure with that your numbers get all out of whack.