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Old August 15, 2018, 08:14 AM   #26
jmorris
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As I mentioned before, there are several datums on a cartridge and chamber (marked with a "B" for Basic on the SAAMI drawings), and these are chosen by the cartridge designer or by agreement among SAAMI members.
As long as we can agree that they are chosen arbitrarily, different for different rounds and are in the end just reference points, I think we agree.

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So your best bet for an unfamiliar chamber is to measure the case to whatever shoulder reference you please, then mark the case shoulder with a Magic Marker and set up your resizing die, slowly turning it in while checking the ink for contact marks to determine when shoulder contact is complete.
That’s where I was going in #21, works all the time, less cost and hard to get confused about the process.

Last edited by jmorris; August 15, 2018 at 08:21 AM.
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Old August 15, 2018, 09:37 AM   #27
F. Guffey
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but the fact this arbitrarily chosen diameter is a reference does not make it a datum.
That can happen to reloaders that are in mortal combat with reloading and other reloaders. I start with the datum, I do not allow myself to be driven to the curb by SAAMI and I do not get confused with thinking ever gage is a head space gage and I do not believe the case has head space. I off set the length of the chamber from the shoulder of the chamber to the bolt face with the length of the case from the shoulder to the case head.

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Reloaders have improperly seated cases with too much crimp. I understand to a reloader there is no correlation between bumping/moving the shoulder back and bulging the case at the case body/shoulder juncture; but the seating die does not have case body support, the full length sizing does have case body support: Meaning it is impossible to move the shoulder back when sizing a case it is possible to shorten the distance from the shoulder to the case head but the shoulder the reloaders starts with is not the same shoulder he finishes with.
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Old August 15, 2018, 02:25 PM   #28
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Originally Posted by jmorris
As long as we can agree that they are chosen arbitrarily…
I'm not sure that is so. I've noticed they like to use the same diameters where they can and, in most instances, they try to avoid numbers that aren't multiples of five thousandths. I also have noticed that most (but not all) of the datum diameters are closer to the outside perimeter of the shoulder than they are to the center of the shoulder, which is where I would have put it. Perhaps its because the case is more rigid there, but I don't know. Considering that SAAMI is a manufacturing standards association, their members probably like to minimize the amount of different tooling they have to buy, so that may be an influence as well. Some of the Weatherby cartridges don't follow that, and I expect it is because of the large transitional radii linking the shoulder to the body and neck.

It seems like a good question to ask SAAMI.
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Old August 15, 2018, 02:49 PM   #29
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On reconsideration, I think part of the reason for the location is they are subtracting the effect of the maximum corner radius at each end of the chamber neck before locating the shoulder datum. The chamber's shoulder/neck corner radius is normally larger than the shoulder/body corner radius so a range of cases can fit without jamming. But this also means that when you pick a location on a fired case to measure, you have to pick a diameter that doesn't overlap the ends of those radii and expect to get a valid setback measurement off your resized case.

In the 223 Remington, for example, the shoulder is only certain to be flat between a diameter of 0.2768" to 0.3514". The exact middle of that range is where I would put it, but that is 0.3141". They chose 0.330". 0.315" would be the closest multiple of 0.005", and they didn't choose that. So I am thinking 0.330" was already used on something else and they decided to give it double duty.

Again, it's worth asking them what the rationale is behind the choice.
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Old August 15, 2018, 08:34 PM   #30
jmorris
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I think part of the reason for the location is they are subtracting the effect of the maximum corner radius at each end of the chamber neck before locating the shoulder datum. The chamber's shoulder/neck corner radius is normally larger than the shoulder/body corner radius so a range of cases can fit without jamming.
Again good information but really not needed for the OP. All of this speculation assumes the tool the OP bought was the same as what?
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Old Yesterday, 09:38 AM   #31
F. Guffey
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It seems like a good question to ask SAAMI.
This forum has at least three members that claim they called SAAMI, that started when they discovered SAAMI. I can only guess the first one was a speed reader and was not in the habit of looking for footnotes and directions for reading drawings. It has been at least 65 years since I took my first map reading class; :read right and then up": Nothing has changed, I studied SAAMI drawings, the chamber drawing has a symbol for head space, the case drawing does not have use a head space symbol.

Speak where SAAMI speaks and be silent where SAAMI is silent: I am sure SAAMI was impressed with all of the qualifications listed by our members, I would not call SAAMI.

I can not provoke most reloaders to push away from the keyboard to make room for a RCBS ROCK CHUCKER. They refuse to measure the amount of cam over for the ROCK CHUCKER. They insist the ROCK CHUCKER cams over because they heard it on the INTERNET. I have had 4 ROCK CHUCKERS, I am down to three; I do not own nor have I ever owned a ROCK CHUCKER that cammed over.

I have at least 11 Herter presses, every one of my Herter presses cam over because they are cam over presses, my ROCK CHUCKERS are non cam over presses. Being a good shot does not make a member an expert at reloading.

Read right and then up.

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Old Yesterday, 10:00 AM   #32
F. Guffey
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I think part of the reason for the location is they are subtracting the effect of the maximum corner radius at each end of the chamber neck before locating the shoulder datum. The chamber's shoulder/neck corner radius is normally larger than the shoulder/body corner radius so a range of cases can fit without jamming.
It can not be done? This stuff is not for everyone; in the big inning I had more confidence in reloaders; and now? I do not.

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they are subtracting the effect of the maximum corner radius at each end of the chamber neck before locating the shoulder datum
I open a drawer, locate my datum and then use it to determine the length of the case from the datum to the case head. For me life and reloading is much more simple because my cases do not have head space and the gage I use to measure the case with is a case gage, it is not a head space gage and it is not a case head space gage because the case does not have head space.

Again: The L.E. Wilson case gage has been with us for over 70 years, there is a chance reloaders could now be running L.E. Wilson 'BUT' before internet reloading the case gage has always been a case gage.

The Wilson case gage is a datum based tool: Get a light, shine the light into the gage to aluminate the inside of the gage, for those that can push their way away from the keyboard can see the datum in the gage; It has a radius, I do not use the light. I use other methods but I have shop skills, I use a transfer method, when using the transfer method do not forget to lube the case.

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