I use the XS Sight barrel mounted rear peep on my GPR. GPR barrels are factory drilled for it. Front is either the post with white paint that comes with the XS or a red fiber optic. I've mounted this sight on a Pedersoli Hawken, an Investarms Hawken, & other barrels with slightly different flats than the GPR with some fitting & everyone seems to really like it. It's small, rugged, & easily adjustable. I've put it anywhere from 2" to 6" in front of the nipple depending on customer preference. It even looks like it could be HC. Doesn't give the rifle a "modern" look at all like some of the tang mounted ones. Everyone that has aged eyes just can't see the old style open sights most ML's come with. Without a peep & a modern front sight I just wouldn't be able to shoot anymore, so I don't understand "clubs" that don't allow them, as they are excluding a big portion of ML shooters. Isn't there a law somewhere that prohibits age discrimination???