Shoulder line or Datum line

[Rick Ratzlaff]

So if I understand things the Head Space is from the case base to the DATUM line on a case re the SAAMI Specs. So when shooters are "bumping the shoulder" they are referring to that dimension. eh?

So a "go-no go" gage uses that same datum line spec?

But some tools use the measurement when the case wall breaks into the shoulder or what I've been calling the shoulder.

So when you professional shooter talk "shoulder bump", are you talking the measurement to the Datum line or the shoulder break line?

 

[mikecr]

You're right about go-no-go gages, but shoulder bump is w/respect to interference.
We bump 1-2thou from interference, and with that an un-sprung bolt should turn freely on a chambered case.

The reason you can't go by specs for a fired case is because formed case angles never really match.
Your chamber may be exactly 30deg for shoulder but the formed case will never be.

 

[Edd]

So if I understand things the Head Space is from the case base to the DATUM line on a case re the SAAMI Specs. So when shooters are "bumping the shoulder" they are referring to that dimension. eh?

So a "go-no go" gage uses that same datum line spec?

But some tools use the measurement when the case wall breaks into the shoulder or what I've been calling the shoulder.

So when you professional shooter talk "shoulder bump", are you talking the measurement to the Datum line or the shoulder break line?

If the shoulder angle of your sizing die is the same as your case, the amount pushed back will measure the same anywhere on the shoulder.

 

[mikecr]

Edd, that will never be.
Fired cases expand and spring back to whatever form they take.
So does a chamber -while under pressure.
The result of this is purely local. It can be measured, but not predicted.

 

[Rick Ratzlaff]

We bump 1-2thou from interference, and with that an un-sprung bolt should turn freely on a chambered case.

So you bump to the transition line from the case wall to the neck? Not the Datum spec measurement?

 

[DJSBRS]

So if I understand things the Head Space is from the case base to the DATUM line on a case re the SAAMI Specs. So when shooters are "bumping the shoulder" they are referring to that dimension. eh?

So a "go-no go" gage uses that same datum line spec?

But some tools use the measurement when the case wall breaks into the shoulder or what I've been calling the shoulder.

So when you professional shooter talk "shoulder bump", are you talking the measurement to the Datum line or the shoulder break line?

You are correct in your statement of how to measure headspace. Which is the case base measured to the datum line.
Head space gauges are made from that datum line dimension. However, not all headspace gauges are using the correct cartridge angle for that measurement. But do touch at the correct datum point line. There are numerous tools that measure headspace but will not give you the same number as another tool or the number on a reamer drawing due to their own length and angle to touch the datum line.

You can use whatever appropriate tool you have that is made for measuring headspace and then measure your go gauge and that will be your minimum headspace number for your chamber. Usually the No GO headspace number is .004 longer than the Go Gauge.

When bumping the shoulder back for a fired case to fit your chamber, you are actually sizing the shoulder down and side wall in as well as sizing the neck to fit your original chamber.

When I hydro form custom brass for my customers, I hydro form the brass to headspace to chamber at the datum line.

DJ

DJ's Brass Service
205-461-4680

 

[GLShooter]

Straight datum line for me on die set up. Really I don't care what the numbers are. My chamber may be 0.002 under or 0.008 over SAAIMI, assuming this is a SAAMI case, and that datam number would float all over the place from rifle to rifle. I take a fired case from my chamber and will bump anywhere from 0.001 to 0.006 from THAT case shoulder measurement depending on what platform/need is required for that ammo SAAMI has enough designed in slop that XXX is never constant. Plus remember that not all of these gauges actually measure the datum line.

Greg

 

[RegionRat]

But some tools use the measurement when the case wall breaks into the shoulder or what I've been calling the shoulder.

There are many cartridge designs where the datum line doesn't sit in the middle of the shoulder, but I can't think of one where it is at the body to shoulder facet, or a tool that works that way.

Is there an example of one that I am not thinking of that runs at that facet line?

ETA: These are the SAAMI specs for cartridges that were sumbitted to that committee. You can see older ones and newer ones, but they will show a place on the chamber and another on the cartridge where there is a reference dimension.

https://saami.org/wp-content/upload...99.4-CFR-Approved-2015-12-14-Posting-Copy.pdf

 

[ronemus]

But some tools use the measurement when the case wall breaks into the shoulder or what I've been calling the shoulder.

Unfortunately, the edge between the body and shoulder will be rounded (not a sharp edge as drawn) and it is difficult and expensive to get a good measurement there; measuring at the datum line on the shoulder is much easier and can be done with simple and inexpensive tools.

 

[joshb]

I miss fguffy! He would have had a great rambling opinion on this!

 

[JFrank]

He would probably say the case doesn't have headspace, you create Headspace and shoulder doesn't really bump. Datum based tools have a flaw in the design.

 

[mikecr]

So you bump to the transition line from the case wall to the neck? Not the Datum spec measurement?

Where the shoulder is sized & how much would depend on each case's form as pulled smoking from your chamber. I only bump 1thou (+1/2thou/-0thou), and I check each case as I go to do that. Often, I'll sneak into it with a lube adjustment. This is with a gizzy, cut with my reamer, and calipers.
I guess what I'm saying is ->It's not gonna matter what any drawing shows.

Why the fixation on shoulder datum?
Is this for some special measure?

You set your chamber with GO/NO GO gages.
Dies? No, that wouldn't actually work. Not with any real world precision.

 

[ckaberna]

I miss fguffy! He would have had a great rambling opinion on this!

What happened to the guff? He was wide open when I first joined, and like a bunch of us, he confused the heck out of me.
I got to the point I actually enjoyed his rants. And miss them.

 

[Rick Ratzlaff]

Why the fixation on shoulder datum?
Is this for some special measure?

You set your chamber with GO/NO GO gages.
Dies? No, that wouldn't actually work. Not with any real world precision.

No fixation...yet. I had opportunity to have a smith drop in the go/no go recently. So I'm just trying to realize and learn what I'm using as my base to set my Redding Type S dies for FL resizing. I have used the Hornady tool for several years and just got the Short Action comparator. But they both measure from different places I just learned. Kinda like having 2 watches. Which one is right or best. Pulling out my hair at 74.

 

[JohnKielly]

No fixation...yet. I had opportunity to have a smith drop in the go/no go recently. So I'm just trying to realize and learn what I'm using as my base to set my Redding Type S dies for FL resizing. I have used the Hornady tool for several years and just got the Short Action comparator. But they both measure from different places I just learned. Kinda like having 2 watches. Which one is right or best. Pulling out my hair at 74.

Neither matters a damn after the gunsmith has handed the rifle to you, except to tell you when the case has been adjusted to what you have deduced is correct fit in the chamber. What shoulder bump is about as previously said is helping to guarantee that a round that you have loaded will chamber in your rifle without force or failure. It's the shooting equivalent of final trimming a tenon so it snugly fits in a mortice.

 

[Metal God]

No fixation...yet. I had opportunity to have a smith drop in the go/no go recently. So I'm just trying to realize and learn what I'm using as my base to set my Redding Type S dies for FL resizing. I have used the Hornady tool for several years and just got the Short Action comparator. But they both measure from different places I just learned. Kinda like having 2 watches. Which one is right or best. Pulling out my hair at 74.

I think the first thing to understand is that you are not "measuring" anything , you are comparing something . Once you get that you can understand as long as you are on the shoulder somewhere and not on either junction ( body or neck ). It doesn't really matter where exactly the comparator engages the shoulder as long as use use the same insert on every case so it's comparing at the same diameter/area of the shoulders .

FWIW , I hope he's doing well but I don't miss Guffey and glad he's gone !

 

[mikecr]

So I'm just trying to realize and learn what I'm using as my base to set my Redding Type S dies for FL resizing.

Ok, now we're directly going to your intent.
This is not a static situation, but dynamic.
If you try to set dies using machined gauges only(static), your final product will be all over the place for the dynamic reasons I've pointed out.

Reloading is always a local endeavor. Books and drawings can only get you so far, and then you take it from there. Metal God pointed out that this is really a local comparison, and that your measuring tool can be anything that works for you. It could a 9mm shell casing and caliper (for you).

What you're comparing here should be shoulder-chamber interference to consistent 1-3thou clearance of it.
So you can fire form your brass in your chamber with little or no resizing of it until the brass will not re-chamber freely. This could take 1-3 firings with only neck sizing. Then you take careful measure of that with a tool of your choosing, and adjust your die to bump back from that ~1thou at a time until just reaching the point where your brass is barely re-chambering freely. You're very near 0 headspace at that point.
Now you can adjust your die for another 1thou, or 2thou (whatever you choose) of bump, measure to confirm it (always with the same tool), and log it. That's your correct bump. You'll confirm your sizing to see it from this day forward. Well, you should,, measure every case,, set your gun next to the bench and chamber every case bumped as you go. Just to be sure.
This video shows how to sneak bullets into just touching lands, but the process is similar to see shoulders just touching chamber.

 

[Rick Ratzlaff]

Thanks guys - really appreciate this. You actually taught an old dog new tricks.

 

[M-61]

I miss fguffy! He would have had a great rambling opinion on this!

And, if I recall correctly (which is impossible regarding him) he either sold or had the only tool on earth for measuring this. Or maybe he sold datum lines! And remember there is NO press that cams over nor anything that can bump a shoulder.
I'm sorry he stopped posting.

 

[perazziguy]

I miss fguffy! He would have had a great rambling opinion on this!

^^^^^THIS! Remember, he is THE only one who really knows what happens inside a rifle's chamber. Or something like that. I just resize my 22-250 brass then use the Lyman case gauge. Not enough, then the bolt is a bear to close.
Jonathan Taylor