If the wheel gun is single action, then who cocked the hammer?
Did He do this also, or was it handed to him already cocked?
If he did both click the hammer and squeeze the trigger, then this requires premeditated action, not merely a light trigger, or negligible discharge.
My theory is that he pressed the trigger as he was doing the cross-draw. Then thumbed back the hammer and either released it thinking it was cocked or his thumb slipped off the hammer.