I really appreciate this discussion. I have two of those primers machined from brass stock, with screw ends and plunger dispensers, and I shall retire them forthwith. A small horn, particularly a flat one, is a great priming device.
This accident reminds me of a minor accident that I had quite a few years ago. In those days, I must have been bringing the cock to full bent, primed the pan, and then snapped the frizzen closed. I was holding my rifle in the left hand, and closed the frizzen with the fingers of my right, and the rifle fired. My right hand was directly in the path of the gases exiting the vent, and I still bear the tattoo, particularly between my fingers where the skin is not so tough. Had I still had the primer in my right hand when I closed the frizzen, ......yikes!!
So now, I never bring the cock to full bent until I am ready to address the target.