There was a magazine back in the 80s named "Firepower", and they occasionally did articles on movie, and tv weapons. In one issue the covered a number of different weapons, including the Rifleman's Winchester. They had a poor photo of the rifle (this was before laser-printed magazines) showing the set screw arrangement. The set screw was at the base of the trigger guard, where it met the hoop. Tightening the screw would cause it to move up into the trigger guard, where it would catch the trigger whenever the lever was actuated. If you carefully watch the opening, you can see that his finger is nowhere near the trigger. If you tried that without some type of arrangement like that, then I imagine that you would end up impaling your finger on the trigger.
Michael Landon used to tell a story about Chuck Connors. They were good friends when "The Rifleman" became a hit, so he asked CC if he would come and put on a show for his Cub Scout Troop, as they all loved the show. CC agreed. The troop met in a school that had a stage, complete with curtain. ML told the the kids that he had a special treat for them...CHUCK CONNORS, THE RIFLEMAN! And nothing! ML went behind the curtain to find CC holding the rifle in one hand, and the blanks in the other hand. ML asked him what was wrong? CC replied that he didn't know how to load the rifle! Michael Landon said that here was a man that could make the rifle sing and dance, but they had never shown him how to load the rifle. So ML showed him how to load the rifle, and CC put on a great show for the Cub Scouts.
David.