So if you find you ARE that friend, presumably you'd have no fear of stepping in front of that gun barrel yourself for a few million flips right afterwards.
I don't believe so. While the person who underwent the experiment has a completely convincing proof of MWI+QTI, the friend doesn't. What he saw is just as unlikely under MWI as it is under Copenhagen.
If you didn't believe in MWI+QTI (i.e. had very low priors for), and you saw your friend claim to do a billion coin flips and not get shot, then even if you "refuse" to significantly increase your belief in MWI+QTI, wouldn't you at least increase your belief in the possibility that the gun is broken, and thus would not shoot you?
In passing, I said:
And lo, CronoDAS said:
To which I replied:
There's a certain resemblance here - though not an actual analogy - to the strange position your friend ends up in, after you test the Quantum Theory of Immortality.
For those unfamiliar with QTI, it's a simple simultaneous test of many-worlds plus a particular interpretation of anthropic observer-selection effects: You put a gun to your head and wire up the trigger to a quantum coinflipper. After flipping a million coins, if the gun still hasn't gone off, you can be pretty sure of the simultaneous truth of MWI+QTI.
But what is your watching friend supposed to think? Though his predicament is perfectly predictable to you - that is, you expected before starting the experiment to see his confusion - from his perspective it is just a pure 100% unexplained miracle. What you have reason to believe and what he has reason to believe would now seem separated by an uncrossable gap, which no amount of explanation can bridge. This is the main plausible exception I know to Aumann's Agreement Theorem.
Pity those poor folk who actually win the lottery! If the hypothesis "this world is a holodeck" is normatively assigned a calibrated confidence well above 10-8, the lottery winner now has incommunicable good reason to believe they are in a holodeck. (I.e. to believe that the universe is such that most conscious observers observe ridiculously improbable positive events.)
It's a sad situation to be in - but don't worry: it will always happen to someone else, not you.