Re: Measuring base of case to bullet ogive
It sounds like Rich got you straightened out with a good descriptive visual there. Yes, the ogive is the entire arc, from bearing surface to meplat. What it sounds like you've got it confused with is what we'd call a datum line, essentially an arbitrary point at which we're taking the measurment along the ogive, for comparison against other individual rounds. That's where the variations I mentioned earlier come into play, especially if you're trying to compare measurements taken with one tool with those taken using a different tool. Little things that'll add up and sneak up on you if you don't keep an eye on them.
Here's another visual that might help; take two or three bushings smaller than bullet diameter, say a .300", a .295" and a .290". Insert a .308" diameter bullet, tip down into the bushing. At some point the ogive will contact the bushing and stop it from moving any further. All three will stop on the ogive, but you can see that all three will stop at different points along the ogive. Those would be the datum lines for those respective measurments, and could be compared against other bullets measured in that same bushing. Hope that helps!