Now you are talking about something I know about! If you are just using it to kill them in a trap, use your single six with the mag cylinder in it. Out of all the ones I've killed in the meat plant, which were done at 3-4 foot in a knock box (like your trap), most didn't fall to a 22lr out of a pistol with a single shot, but with a mag shell, they did. I don't know how your set up is with traps, but a .22 rifle will work wonders, as it has much better penetration than most would think. I've found the bullet out of a rifle in a foot into a steer's neck when cutting them up(about 18-20ish inches penetration). Will a .357 work on a trapped hog?....why hell yes it will, but if it was me, I would use a .22 rifle, or your single six using mag shells, and don't forget to bring the ear plugs.
~~~Amos