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.

 

[LCazador]

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.

As long as you make a note that you measured with comparator 'A' or 'B' it really doesn't matter. You measure bump so headspace is consistent. The best way is to measure is to have a gizzy made from a barrel stub with the same reamer that is used for your chamber. Even then you should make a note of what you used to set shoulder bump. If you use competition shell holders also make a note of what shell holder you used to achieve your desired bump. Remember that this measurement is dependent on brass springback.

 

[ORjeff]

I make a comparator like tool that does allow you to measure from the actual datum line. Does a couple other things too.

Cartridge Gauge

Cartridge Gauge

superior-precision.com

 

[chop house]

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?

As guffey woulda said. You can't move the datum, you are pushing the base forward.

 

[divingin]

"Datum" is simply a generally agreed upon reference point. SAAMI usually has it listed as halfway down the shoulder for a bottleneck case, but that does not need to be where it's measured, as long as all measurements are taken with a consistent criterium. As long as the measurement spot will identify errors, it should give the results as intended.

Here's a wiki on the term:
>>>>>>>>>>>>>>
"A datum reference or just datum (plural: datums<a href="https://en.wikipedia.org/wiki/Datum_reference#cite_note-plural-1"><span>[</span>Note 1<span>]</span></a>) is some geometrically important part of an object, such as a point, line, plane, hole, set of holes, or pair of surfaces. It serves as a reference in defining the geometry of the object and (often) in measuring aspects of the actual geometry to assess how closely they match with the nominal value, which may be an ideal, standard, average, or desired value.

For example, on a car's wheel, the lug nut holes define a bolt circle that is a datum from which the location of the rim can be defined and measured. This matters because the hub and rim need to be concentric to within close limits (or else the wheel will not roll smoothly).

The concept of datums is used in many fields, including carpentry, metalworking, needlework, geometric dimensioning and tolerancing (GD&T), aviation, surveying, geodesy (geodetic datums), and others."
<<<<<<<<<<<<<

In a practical sense (and repeating a lot of what was said above), say you fired a cartridge in your rifle, and you want to know the change in length that occurs. You measure a fired case and an unfired case. The SAAMI datum point gives you a difference of, say, .003" from base to the point measured (halfway up the shoulder.) If your measurement on both cases is taken 5 thousandths higher or lower than the SAAMI datum line (and you assume that the shoulder angle remains the same), you'll still end up with that difference of .003". It only means your "datum" point or line is different than SAAMI's; not that it is more or less useful than SAAMI's. The actual measured length will be different, but that's moot, as it's only the difference that you're concerned with (in this case.) Comparators are intended to compare, not to measure.

 

[PhilC]

I thought Sincair hex comparators were for BTO or CBTO measurements both of mine are. Don't recall if Sinclair also made a hex version for case measurements, possibly so.

They did make a comparator set to use with calipers that required different caliber bushing inserts, those I have.

 

[209jones]

Used the Hornady set for a while, then got the Forster unit, few yrs later I bought an extra body for the Forster to have one set up for case and one for CBTO. Like the Forster as all I have to do is loosen it and spin up the hole I want, no searching for the right bushing to use. All a person is doing is creating a reference point to work from, what tool you use is up to you, but as mentioned by others, use the same tool all the time, don't mix two brands.

 

[chop house]

measures from the exact datum line.

For the youngsters amongst us, guffey's did.

 

[chop house]

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

It would seem, since we are trying to determine results under a thousandth, that a partially seated virgin cup would be better. Even then I can imagine issues measuring, and adjusting the die. Practical application might be difficult, but an interesting idea though.

 

[JGSprecision]

As somebody who has spent countless hours measuring gauge datum lengths and having to have those measurements certified by accredited metrologists (the sounded official)-
Measuring with a 90deg face datum ring gauge has never been repeatable.
Too many factors will end in sizable errors.
IMHO a method I would trust would be base your measurements on a GO gauge w/ an shoulder angled headspace comparator.
This gives a solid reference point that can be verified in barrel/bolt anytime. In practice its as easy as loading the HS gauge , zero and it's ready to check.

Case gauge is similar but you can't use a CG to check the actual requirement without another gauge or something used as a gauge. Still a great tool and a great substitute for a comparator.

 

[VA_XTC_Shooter]

I verified the whidden gage measures 0.000 on a headspace go gage, so id say its the closest one to a real number you can compare numbers with a buddy vs the other comparators.