Shoulder back or Datum line back?

[Rick Ratzlaff]

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?

 

[DShortt]

Yes. 'Bumping the shoulder back' means the brass case will be shorter when measuring from the base to the datum line.

 

[Rick Ratzlaff]

Thanks I was sure about that, but needed to confirm my old mind. Thanks again.

 

[piie]

Ye, if you want to be technically correct it’s the datum line- but I am not aware of any tool out there that will guarantee you it measures from the exact datum line. Most of us measure from “a line” around the shoulder slope to the case bottom. As long as the line is consistently on the shoulder “somewhere” the comparative measurement will still be sufficient in telling you when you moved the shoulder back sufficiently. It works because the shoulder is a set angle and the die will move it all, evenly.

 

[DShortt]

Ye, if you want to be technically correct it’s the datum line- but I am not aware of any tool out there that will guarantee you it measures from the exact datum line. Most of us measure from “a line” around the shoulder slope to the case bottom. As long as the line is consistently on the shoulder “somewhere” the comparative measurement will still be sufficient in telling you when you moved the shoulder back sufficiently. It works because the shoulder is a set angle and the die will move it all, evenly.

Agree. That's why the measuring tools are called comparators. You compare a distance reading of different cases or before and after sizing.

 

[Webster]

Agree. That's why the measuring tools are called comparators. You compare a distance reading of different cases or before and after sizing.

I thought only Guffy used the term comparator.

 

[Coyotefurharvester]

Or you push the shoulder tight to the chamber wall using the spent primer method, measuring the actual bolt face- cartridge head gap.

 

[DShortt]

I thought only Guffy used the term comparator.

 

[Rick Ratzlaff]

Ya, interesting I bought comparators from 2 different suppliers. And their different. I was ticked off. I assumed that the Datum line would be the SAMMI spec. My biggest problem is, I don't know what I don't know.

 

[64Rambler]

Ya, interesting I bought comparators from 2 different suppliers. And their different. I was ticked off. I assumed that the Datum line would be the SAMMI spec. My biggest problem is, I don't know what I don't know.

Nope. My Hornady comparators give me a different values than my Derraco comparators. I also have two of the "same" Hornady comparators and they differ in indicated value by 0.001"-0.0015", so I have them marked A and B and make sure I know which one to use for which measurement. I use the A for CBTO and the B for the Boat tail with A when measuring bearing surface.

 

[JFrank]

Ya, interesting I bought comparators from 2 different suppliers. And they’re different. I was ticked off. I assumed that the Datum line would be the SAMMI spec. My biggest problem is, I don't know what I don't know.

From my perspective It doesn’t matter where you measure along the shoulder angle, you can create your own datum line.

 

[Rick Ratzlaff]

From my perspective It doesn’t matter where you measure along the shoulder angle, you can create your own datum line.

HMM Interesting. If so, is that the engineering point that you calculate your Shoulder Bump from? If so, why not have a gage that measures to the shoulder in the SAMMI Spec (+/-) Sounds like I'm over thinking this.

 

[DShortt]

HMM Interesting. If so, is that the engineering point that you calculate your Shoulder Bump from? If so, why not have a gage that measures to the shoulder in the SAMMI Spec (+/-) Sounds like I'm over thinking this.

It comes down to cost. If you want a measurement tool that is EXACTLY the datum line diameter and is made from material that will not wear with use, it'll be costly. So. Inexpensive comparators are made and in reality, it doesn't matter that much. You just need a 'before' and 'after' measurement. The brass will never completely fill the chamber and remain that way after firing. So it's just being 'fussy' to insist on an exact datum diameter tool.
Might not be the answer everyone likes, but there it is.

 

[357Mag]

I thought only Guffy used the term comparator.

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

 

[L.Sherm]

Whatever your gonna use just use it everytime don't be switching up comparators.

 

[joshb]

I miss FGuffey! He’d type up a 300 word response that NOBODY could fully understand!
Just use the same tool everytime. Stupid me bought a back up set! Yup! They’re different and useless with my old records!

 

[William Vogel]

I don’t think it matters if you always use the same one as long as you always use the same one for each batch of brass. I measure my fired brass and then size it in the die. I don’t worry about memorizing or even having the exact number written down because it’s that I want this brass to be 0.002 shorter after I size it. By the way I don’t adjust my die unless I start a new batch of brass. I haven’t adjusted my die in a while.

 

[DShortt]

I miss FGuffey! He’d type up a 300 word response that NOBODY could fully understand!
Just use the same tool everytime. Stupid me bought a back up set! Yup! They’re different and useless with my old records!

I don't miss Guffey, though. Always seemed to me that instead of explaining things simply, he intentionally tried to over - complicate things.
And for what? Waste the time of others and appear more knowledgeable?

'I got stuff to do. Gotta go.'

 

[joshb]

I don't miss Guffey, though. Always seemed to me that instead of explaining things simply, he intentionally tried to over - complicate things.
And for what? Waste the time of others and appear more knowledgeable?

'I got stuff to do. Gotta go.'

Yes, at first it was frustrating, but then (to me) it was like watching a good comedy sketch! A mix of Archie Bunker, Kramer from Seinfeld and Monty Python.

 

[DShortt]

Yes, at first it was frustrating, but then (to me) it was like watching a good comedy sketch! A mix of Archie Bunker, Kramer from Seinfeld and Monty Python.

I can appreciate that perspective. It just wasn't my 'cup of tea'.
I prefer to get right to the point, and Guffey was anything but directly on point.
But, that's what makes this forum what it is. Something for everyone.