OK, I'll jump in on this one. I've made a bunch of 5x5s and used them for marten, mink, and raccoons. They're deadly on all three. For 'coons, the big thing is avoiding triggering with the paw. My traps have a striker bar on them and they will hold a 'coon by the paw but there's too much room "under the jaws".
To avoid paw catches, I put the trigger on top on the inside jaw and bend it so it's almost horizontal. I also have holes in the tops of my boxes and wire the bait to the top. Occasionally I'll have him caught by a paw and the head, but he goes down quick. My springs are made of 3/16" music wire and the striker breaks the spine about half the time on coons (always on mink and marten). Deep boxes, mail boxes, or buckets are a big help to reduce reaching.
As far as 160s vs 220s. I think 160s kill 'coons quicker than 220s. Like RayA, most of my 160-caught 'coons just fall down. I think 220s catch them a bit further back and they don't get stunned like they do when the 160 (or 5x5) raps them in the base of the skull. That being said, I got a lot of avoidance trying 160s in trail sets- even when they were lifted up off the ground. Now I just use buckets or boxes right next to the trails. They rarely seem to get past the bucket. ;-)
I bait my conibear sets with marshmallows and sweet baits/lures to avoid non-targets. I save the muskrat meat or fish for the dirtholes and pockets with footholds. So far- so good. 34 seasons with no dogs caught.
Michigan Trappers and Predator Callers Association