So, this level of technicality comes down to specific SA revolvers. I have not seen this MythBusters episode, but to the best of my knowledge, both today and 150 years ago, some SA revolvers function as you say where a trigger pull is needed each time, and some basically do not.

As a kind of example, I used to have some kind of 50s era Ruger SA revolver in .22lr.

Now I never actually tried fanning at a range, (can you imagine most places will ban you instantly for this as its generally extremely inaccurate lol?) but with the cylinder unloaded and completely clear… basically, if you pulled the hammer back, pulled the trigger, click, hammer fires, or would have fired.

But then, if you pull the trigger back just a bit more, then I could work the hammer with my other hand. While pulling back the hammer, as the cylinder rotates, the trigger fights back and wants to reset just a bit, but once the cylinder is lined up, the trigger has reset, and i am still pulling on it, and click it ‘fires’ again.

So… its not really pulling the trigger separately for each shot, its more like pulling it firmly, and while you work the hammer, the trigger pushes back a bit, but if you keep the same grip, it does just fire again.

I am aware from researching online and asking old timers that this level of action manipulation varied amongst SA revolvers of old.

With some SA revolvers the trigger reset is much more pronounced such that as you say, its pretty much a totally new pull of the trigger each time.

Either way, in the real west, fanning, while possible to do with some pistols, was extremely uncommon in basically anything other than showy demonstrations…

However, Ruger now makes a model called the Vaquero, which is specifically designed for the fanning competitions I mentioned earlier.

And given that these competitions have been around for at least a decade before than thing hit the market, I would be willing to bet that other SA revolvers could properly fan, maybe you might have had to get a bit of a custom trigger job or something though.

Edit: Fun possible fact, I am fairly, though not totally sure that the term ‘hair trigger’ actually comes from these kinds of SA revolvers that either naturally or via tinkering had such a small give and reset that it was said to be the miniscule distance of a literal hair, and thus more easily allowed rapid fanning.

Though as with many gun related aphorisms, that origination of the phrase may be debated/apocryphal.