Although I don't use slings on shotguns, I have always used the barrel down carry for rifles. I'm right handed, so I carry the rifle on my left shoulder and it stays put. The weight is all below the carry point, the barrel won't snag and I can grab the forend quickly and swing it into position. The buttstock is behind the left shoulder, with the underside of the rifle faceing foreward, if you're having trouble visualizing how it's done.