i take it you are refering to the projection of a three dimensional object on a plane surface...?
From the beginning I knew the datum was not a line, I knew the datum was a round hole, I knew one datum diameter was .375" in diameter, it was clear to me the diameter of the hole did not matter if I was using a comparator. And there are case friendly datums and there are datums that are not case friendly, I am the fan of the non friendly datum, I understand in the reloading world there are too many excuses for not being able to do something. If I check a case with an unfriendly datum the case will have a circle around the shoulder, to see the circle when using a case friendly datum I have to use a hammer, or rotate the case like I was attempting to get it hot enough to build a fire. And there is Persian blue, smoke and markers. I can see the datum from the front, side and top, That is how the old wood carver became a land owner, the Duke gave the wood carver half of his Dukedom.
not particularly so. I think most reloaders believe the entire case moves forward, and then as a result of firing the case head moves backward. Therefore neither the shoulder nor the datum line (whether round or plane, your choice of reference) moves.
I have heard that, it goes back to the reloader that claimed the firing pin drives the case forward to the shoulder of the chamber; once the case shoulder contacts the chamber shoulder the primer is busted, he claimed. I suggested there were events that he was omitting etc.. so I disagreed with him. As always, he got all potty mouth/social media (potty finger).
I tried to tell him I had killer firing pins, I explained to him my primers were busted before their little buddies, case, powder and bullet knew the primer was hit. In my opinion failure is designed into the theory the whole case moves forward and then when fired the case head moves back while the shoulder of the case remains against the shoulder of the chamber. BECAUSE the only way that can happens is if the case stretches between the case head and case body. I gave an example: I fired short cases in a long chamber. The chamber was .127" longer from the shoulder to the bolt face than the case from the shoulder to the case head. The firing pin had no trouble catching the primer as it raised for the front of the chamber
I think most reloaders believe the entire case moves forward
. My cases did not stretch .127" between the case head and case body; Proof? had the case stretched between the case head and case body the length of the case neck would not have changed (the length of the case from the shoulder to the case head would have changed). When I ejected the cases there was only a hint of a neck beyond the shoulder. And then there is that argument about the shoulder moving, the shoulder on my case when fired did not move it became part of the case body, when the case formed to the chamber a new shoulder was formed; one more time the case did not stretch between the case head and case body.
Hatcher did the same experiment, years later I had a friend that built 4 magnificent rifles, he made the wildcat reamer and when finished he headed to the range. He had 5 case head seperations out of the first 10 cases he fired, he did not use the same rifle Hatcher used, he did not use the same rifle/receiver I used, I did tell him he could have determined if that would happen before he left the shop and I told him I could have met him at the range and fixed the problem long enough for him to form his cases and I told him I would have formed the cases first and then fired.
F. Guffey