I have a roller guide on my Tippmann, and it works very well. On my other machines I usually just use a Yale-type key, taped to the arm with masking tape. I put the needle through a piece of scrap leather at the appropriate distance from the edge, slide the rounded end of the key up against it, and secure in place with a couple of wraps around the arm. I don't use the Yale key for any particular reason . . . we just have lots of them laying around, and they're about the right thickness!