Re: Bumping the shoulder on a loaded round
Firsthand experience with a few hundred rounds I just didn't want to break down.
I had to use a body die, because the loaded rounds just won't fit into the neck region of a regular die with a bullet in the neck.
There's really nothing going on which would light up the round.
Now, if you put a tiny steel ball bearing of maybe .05 right in the middle of the shellholder, and could fit/keep it under the round while it's in there, and THEN slammed the ram up, you might be able to get a primer to fire I'd guess (note that special word) that you might be able to get 1 out of 10 or 25 to fire. Because the die is just like a chamber, any round lighting up would have full confinement and would be expected to generate some impressive pressure really, really fast.
So, my experience reloader "spidey sense" says that there's hardly any likely way a round could get fired while being bumped back, but if one ever did, it would be catastrophic. I have no idea whether the steel of our reloading dies is suitable for use as a chamber, or is too brittle, or is too soft (I have reasons to doubt too soft).
As for my experience, just remember the joke about the hitchhiker and the driver who slammed on the brakes when they finally reached a green light. I might be that driver's brother-in-law....