You obviously have been owned or fired one of those old guns. The cylinder is aligned under the hammer when it falls. Why because cocking the hammer back advances the cylinder and locks it only when the hammer is fully back it does not advance it on the way down. If you pull the hammer to half cock it only advances the cylinder less than half way as half sick is only a term it’s not literally half cock it’s more like a quarter. If the hammer is released from half sick the cylinder will rotate backwards to it’s original position under the hammer as neither the advancement paw nor the catchment sear has been fully engaged until the hammer is all the way back. I’m not going to argue this is 140 year old proven problem for these types of guns. YouTube it there are a dozen examples of single action going off half cocked or the older models don’t even have half cock sears if you let the hammer go at any point before fully cocked it will slam back and fire a round. Look up thread I already posted a video of one of these guns going off without the trigger even being touched. I own two of these types of revolver both will go off of the hammer is let go from half cock fogger no where near the trigger they completely lack transfer bar or pin safety they have live pins sitting on live primers. This is the reason all modern revolvers have pin safety mechanisms. Google it you will find dozens of videos of this issue.
I am holding my UNLOADED colt 1851 in my hand right now. What you describe CANNOT happen. I am trying to do it. I am doing it now.