He had to have pulled the trigger.
L
So here is a question I can’t answer because I’ve never handled a Colt SAA or replica with no transfer bar. If you cock the hammer without your finger on the trigger, but let go of the hammer before it engages the full cock notch, will the hammer fall all the way back and strike the firing pin?
I don’t know. And that still doesn’t explain why there was a live round in the gun.