

There was an adjustable screw added to that rifle's lever handle to trip the trigger at the end of cycling the action. His trigger finger never had to be inside the loop, for fear he'd empale his finger on the trigger during a rapid succession.
Notice in the opening shot, he doesn't do the iconic "flip" until he's out of ammo and starts reloading. If he flipped the rifle with a round in the tube, the screw would cause it to fire at the end of the flip.

Also, the opening shows him firing 12 fast shots, even though the rifle only holds 10 rounds.
Hollywood magic!
The majority of the rifle skills demonstrated were just that -- skills. Chuck was an athlete before going into acting, and he was able to manage nearly all the amazing things asked of the character.
Of course, the one thing he excelled at was time travel. The series was set in the 1870s-1880s, yet his rifle was a modified Winchester 92 -- as in 1892.
