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[Son]

IIt's all in the position and motion of the thumb doing the stripping. Away from my pics at the moment, but if you practice with your thumb as close to he charger as possible you will succeed every time. Then try stripping the rounds with your thumb a little forward and you will get rim over jams.
As for it happening with the last round in the second charger, that is likely because you are using a second effort, and not keeping your thumb back.
Stick your rifle in a rest or stand and stand over it watching what happens when you strip the rounds. You cannot see what happens with the butt in your shoulder on the mound, you need to understand that the round entering the magazine moves back as it leaves the charger making sure the next goes in front.... If you do it right!

Look, learn, practice and it won't go wrong on the mound again.

Will stick up pics later when I get home.