So if I undergo the classic 50/50 decaying radioactive particle + gun experiment, it would seem to me that I have a 50% chance of my consciousness surviving and a 50% of it going ping out of existence when the bullet pulverises my brain.
Exactly. QI just doesn't work many would like it to work. Consider "Quantum Immortality Lite", where you load the gun with a sleeping pill instead of a deadly bullet. This version is easier for humans to visualize, because it involves no "mysterious" (previously not experienced) phenomena, such as permanently ceasing to exist. The line of reasoning is the same: awake copies of the experimenter will notice that he never seems to fall asleep, but you will be happily snoring on the floor with near certainty if you pull the trigger ten times. It won't always jam for you.