Sinclair bump gauge error

[o2bmark]

While working up a load for my Christensen Arms rifle I found the disparity between datum line measurements and Sinclair tapered insert measurements for setback amount. The problem shows up on FL sized cases where not only is the shoulder set back but also the diameter of the body where it meets the shoulder is reduced apparently allowing case to go further into the insert/die. This is misleading as to actual setback and gives a larger value. If the Sinclair insert taper were to accurately mimic the angle of the shoulder and sit flat on it then in theory the diameter of case would not influence the reading. What seems to be happening is the die is measuring at the radius (corner/edge) from shoulder to body of case. I'm using #20 insert on 308win case with Redding FL dies. With my FL die setup the difference between fired and resized setback is .002" compared at several different datum diameters. Sinclair indicates .007" of setback. The case diameter just before Shoulder is reduced .002" from fired.

 

[Mikecr]

It's possible that the case shoulder angle isn't what you expect.
It's possible that the die & it's setting is not bumping shoulders, but purely reducing shoulder angle instead.
Zero HS could be reached & cleared with a given chamber:
-At the neck-shoulder junction
-At the body-shoulder junction
-With shoulder angle
-Various in summation
This, depending on your die/chamber/brass match, or mismatch.
Do you know how shoulder bumping is supposed to occur?
If you feel like you have a combination that is causing an excess, that may be valid, but what will you do about it?

Maybe it really doesn't matter so long as it is a consistent smidge from a zero head space condition.

 

[o2bmark]

I checked setback with hornaday, Sinclair, and by cartridge base to bullet base of bullet sitting on lands and only Sinclair measured .006-7 while other two .002
I've been loading for more than 50 years and understand headspace. The perfection of the chamber or my Redding dies has not been measured by me but the difference of how far a fired and resized case enters the chamber is what I have measured. I blackened a resized case and pushed it firmly into Sinclair insert. The heavy contact points were where neck meets shoulder (surprise to me) and shoulder meets body (expected). The flat of the shoulder was marked but much lighter indicating that insert was reasonably accurate on mimicking angle. All of this could explain the different measurement but I would only expect by 1 or 2 thousandths not 5. Now that I've examined the blackened case my suspicion is that the neck to shoulder joint is the problem. Redding dies use neck size bushings which are beveled and cannot get but so close to shoulder perhaps leaving enough unsized neck to skew reading. I may bevel the Sinclair insert at neck just to see what happens. It's all just curiosity on my part from this point forward Since I have other ways to measure.

 

[MagnumManiac]

I am confused as to how you determined WHERE on the case is measuring from.
The HS datum, which is measured at half the length of the shoulder, regardless of angle, is the ONLY relevant place.
I read your post several times, how does measuring case base to bullet base to lands/rifling tell you anything about headspace, the chamber IS going to be different to your brass measurement anyway…
I need more information to understand your issue correctly.

Cheers.

 

[o2bmark]

I am confused as to how you determined WHERE on the case is measuring from.
The HS datum, which is measured at half the length of the shoulder, regardless of angle, is the ONLY relevant place.
I read your post several times, how does measuring case base to bullet base to lands/rifling tell you anything about headspace, the chamber IS going to be different to your brass measurement anyway…
I need more information to understand your issue correctly.

Cheers.

The only thing I really care about is how much clearance I have on chambered rounds. The headspace measurement is to determine how much front to back clearance or movement the loaded round has. .002" is what I try to get in order to allow reliable chambering with minimal Play (hunting rifle). When Measuring a datum line I don't care exactly where it is as long as it is the same datum since I'm only interested in the difference between cases not the reading. No matter where I measure, top of shoulder, bottom of shoulder or middle of shoulder, the difference between fired and resized has always been the same. On the other hand the Sinclair insert die is cut to touch the entire shoulder, not a datum line—looks like the very top of a resize die between neck and body. It really needs to match case shoulder precisely.
The base to base measurements are what I do to determine COAL to lands (really case base to bullet ogive) for setting freebore/jump. In the process I measure where the case base is on a chambered case. New case goes in chamber further, resized goes less and fired case least. This means I'm actually measuring how much further a resized case goes into chamber which is my true goal.

 

[mudflap621]

I checked setback with hornaday, Sinclair, and by cartridge base to bullet base of bullet sitting on lands and only Sinclair measured .006-7 while other two .002
I've been loading for more than 50 years and understand headspace. The perfection of the chamber or my Redding dies has not been measured by me but the difference of how far a fired and resized case enters the chamber is what I have measured. I blackened a resized case and pushed it firmly into Sinclair insert. The heavy contact points were where neck meets shoulder (surprise to me) and shoulder meets body (expected). The flat of the shoulder was marked but much lighter indicating that insert was reasonably accurate on mimicking angle. All of this could explain the different measurement but I would only expect by 1 or 2 thousandths not 5. Now that I've examined the blackened case my suspicion is that the neck to shoulder joint is the problem. Redding dies use neck size bushings which are beveled and cannot get but so close to shoulder perhaps leaving enough unsized neck to skew reading. I may bevel the Sinclair insert at neck just to see what happens. It's all just curiosity on my part from this point forward Since I have other ways to measure.

If I'm reading correct you are comparing measurements from different comparators? If so this is not the way, no two are the same. It's only a relative measurement so pick one comparator and stick with it.

 

[Mikecr]

Whenever dealing with angled datums, measurements (as intended) can get really tricky.
I believe there is a learning opportunity here, at least for me.

 

[o2bmark]

I found the issue with the sinclair comparator with my rifle (Christensen Arms CF barrel 308win) and my dies (Redding FL, Redding calibrated shell holder set) and have not tested other combinations of caliber and rifle.
The issue is interference from UNSIZED neck of fired case as they come from the rifle. By experimentation I found that the point of first contact with comparator was on the slope up from shoulder to neck which on the raw fired case was .346+" in diameter (wow, really loose/large chamber in neck!). My sized neck is .3360" in diameter. Nine thousandths play in neck is ridiculous (loaded round .337"), I prefer 2 to 3 thousandth in non-match bolt gun. If I only neck size a case to .336" leaving the body unsized then measurement with Sinclair system is consistent with all of my other methods of measurement. The error previously found was in the measurement of the fired case as it came from the rifle. Once the neck had been sized all measurements were consistent. When I resized several cases using a different Redding shellholder which stepped case insertion into die in .002" increments the Sinclair comparator showed .002" increases in headspace as it should.

Conclusions: The insert is too tight in diameter at the neck shoulder juncture for MY RIFLE. Given the overly large neck diameter of my rifle it may not be a problem for fired necks smaller than .3465". It certainly works with .336" necks. SAAMI gives .3442" for chamber neck diameter at the mouth and .3462" at the shoulder and my loading manuals show .344".
Additional lesson learned was that my precision aluminum turret press had more spring than I thought (requiring more cam over on shell holder than I like to insure full insertion into size die) and for now I'm going back to RCBS O press for FL resizing chores,
My rifle is a sub 1MOA gun reliably although I seldom shoot more than 300 yds with it. For open country and greater ranges I go for a bigger gun.

I apologize for not doing thorough testing before first posting but hopefully results may help someone eventually. So glad you guys are around.

 

[Mikecr]

Maybe we could conclude that our shoulder bumping measure does not really represent actual headspace settings.
It hasn't been a problem for me yet, but there is potential for it.