Sam was a reasonably active member (more so then me) of THR.us. He's posted there and on .org for a while. In addition to talking about gun stuff he started posting links to Dave Workman's examiner page after .us staff asked Dave not to link to his sponsored site. I might be off a little on the details, but that's basically it.
If I had to guess, GRM is not getting much traffic, so Sam is looking elsewhere for folks to talk to about guns and politics, which is still dicey on THR. But he's a real poster and has talked quite a bit on .us.
I usually find Dave's column to be kinda so-so, with him stretching to make everything a gun-rights battle, so I confess I stopped reading the links unless the synopsis seems really interesting.