First, let me say that I always wear eye protection when loading or shooting – you cannot replace an eye(s).
As mentioned above, I’ve “carefully” deprimed live primers, mostly from split cases that I catch when checking powder level in the Dillon 550 – don’t normally catch upside down primers when using the 550 until the round is complete. I reuse the decapped live primers in practice ammo and have never failed to have one go off.
Only experience with primer detonating when being seated was 50 yrs ago and involved an original Lee loader where you stuck a steel rod in the case and used a hammer to seat the new primer. Was startled by the noise, but the seating rod didn’t move at all in the case.
Recently, I managed to fire a backwards seated primer – first one in over 50 yrs of reloading. When I dropped the hammer, I couldn’t figure out why no recoil, no noise, and smoke was coming from the rear of the gun. Knew something wasn’t right, and wanted to make sure a bullet wasn’t lodged in the bore. I stopped, tried to cock the gun (to get to half cock to remove the cyl), and the cyl was locked up. Thought I had a bullet lodged between the barrel and cylinder. Grabbed the squib rod I always have in the range bag and measured the depth of the bullet in the barrel prior to deciding on a course of action. Turns out the bullet was still in the case – further inspection revealed that the primer had backed out and was lodged against the recoil shield. When I finally managed to turn the cyl and unloaded the offending round, sure enough, the primer was seated backwards and had detonated when hit by the firing pin.
FWIW,
Paul