Re: bullet jam
For all you guys making head to bullet ogive datum (hopefully at bore diameter) measurements to the nearest .001-inch, note how the shoulder position effects the point where the bullet just touches the rifling.
Head to shoulder datum (measured by various gages) will vary by a couple thousandths of an inch across a batch of cases. More with some cases if they've been body sized and shoulder's set back with varying amounts of lube on the case.
And remember that spring loaded ejectors will push the case forward into the chamber where it stops with its shoulder against the chamber's shoulder. And when the firing pin strikes the primer, that's going to drive the case's shoulder harder against the chamber's shoulder. Sometimes the shoulder will set back .001-inch or so. I've reprimed the same empty case, popped its primer then measured its headspace, several times, noting the shoulder moving back a tiny bit each time; proof that this happens.
All of which means the shoulder datum to ogive datum is probably the best way to get the bullet to mate with the lands with whatever dimension desired. Now if someone would just make a gage to measure that with; it's gotta measure between two forward sloping parts.
As for belted cases; they don't have quite the same situation.