Originally Posted by Bart B
Best way for me to measure case head squareness is to first decap it, then stand it on a flat surface against a V block of some sort. Spin the case and watch the case mouth. If the case mouth spins about its center, the head's square. If it moves around in a circle, the size of the circle indicates how much out of square the case head is. Do the same thing with live ammo and watch the bullet tip. Standing the case/round under a fixed magnifying glass makes it easier to see the top make a circle when the case head's out of square.
Congratulations on being one of the few that knows the back end of a case gets pushed off center in chambers by and in the direction extractors' force. Do you know where the shoulder of the case is when it fires? Most folks think the case rests in the bottom of the chamber.
I would say if the neck is not centered on the case axis, which I often find to be the case in new brass, then you could see the same wobble in the neck, even if the head is square to the case body. I am also referring to brass that has minimal neck thickness variation, which with a keen eye, might be seen as neck wobble.
I could not really use the setup you describe, because a non-flat case head may have a surface like a potato chip, rocking on its bottom, making it impossible to apply consistent force to the side and down while rotating and observing. Perhaps my measuring skills are just not up to snuff. I found using the case body held straight in the lathe and measuring run out on the case head to .0001" work better for me, even if the case head is a much smaller dimension to measure.
I am trying to figure a way to use a piece of bar stock in the lathe, with a hole bored down the middle so the case head can rest flat against the bottom. It may be cocked to a tweaked case head, but that's fine, that is what we are measuring. I just need to find a way to hold the case in whatever position it seeks. Then I can use a test indicator to measure how much the case wobbles.