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.

 

[gdbleb]

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.

Agree completely Boyd. I was just trying to correlate my comparator readings to a known, fixed reference. More out of curiosity and learning than anything else. In my modern high-end gunsmith chambers the readings are very close. Older military rifles are a wider range.

 

[big5ifty]

I found that removing the comparator from the vernier and putting it back gave a number different by a few thou.

I also found that if the neck is sized all the way to the shoulder junction, the shoulder moves forward compared to if the neck is only sized partially - in a full length die. I confirmed this in all my four calibers. The effect is larger the bigger the case.

 

[Oklahoma Jim]

I'm surprised that no one has mentioned the Wilson case gauges and micrometer measurement tool. I use them and I find them to be much easier and repeatable than a caliper.

 

[Rickhem]

Webster -

Fred Sinclair offered a machined hex nut-shaped device that featured 6 access holes that covered 6 different caliber’s bullets. He called it a bullet “ comparator “, and it was used in-conjunction w/ a vernier caliper; when measuring ( example ) bullet seatied depth into a cartridge.

I’ve still got an original GEN I that I still use.


With regards,
357Mag

I have one of these and use it for two purposes. First, I use it to determine my cartridge base to ogive measurement. That helps me know if I'm twenty thousandths off the lands, or if I'm jammed ten thousandths. I believe this was the original purpose of the device, and in conjunction with a caliper (as referenced above) it does a great job.

You can also use it to determine how much "bump" you set your sizing die for. For example, I use the 30 caliber "hole" to measure fired brass, and use that 30 caliber orifice to provide the datum line. Run the brass through the sizing die, and measure again. If it's shorter, that's how much you "bumped" the shoulder. You can set your die to provide the specific bump you want. That Sinclair hex nut thing provides a very reproducible measuring point.

I have never seen anything that specifically identifies where on the shoulder the datum line is (or should be) located, so for me, using the 30 cal hole on a 6mm case mouth works fine.