So to prevent this meltdown at the worst possible moment
one should--

1)Make sure all ammo goes kerplunk fully into EACH double chamber
2)Make sure the chambers are clean and free of anything to impair cartridges
3)Make sure the action closes and the action lever closes fullly without impairment(could there have been a component of grease/grit affecting this new guns follower/lever?)
3)Make sure the rifle fits--Work on proper grip and shoulder mounting technique until it becomes instinctive
4)Shoot, open,load, and close the double at speed.

Anything else for a Double man to practice and carry away from this mishap? I really don't want to have to worry about things like this--