I like the third video the best, I think he did a very good job explaning head space and stretch
in laymans terms, very good I thought.
basics : all metals have a percentage of elongation, Elastic and plastic state. once a metal goes beyond elastic into plastic state, it has yielded.
streching to far it goes beyond it's ultimate tensile stress and it fails.
Instead of using calipers as a height gage, I would use an indicator with a stand on a surface plate.
check the measurement over the gage, or micrometers are more accurate than a caliper, not necessary but that's me.
and if any one disagrees with the videos do please pick it apart. won't hurt my feeling and is good for discussion.
but keep it with respect, when people attack instead of rational discussion, you are the one losing respect.
PS I saw suggestions of using a pick tool to check the inside of the cartrige, if necking (undercut) is felt the cartridge is about to fail,
it has reached it's ultimate stress level, I would suggest to toss it away, better to be safe.
all be safe..