Last year I blogged a photo of the prototype of the Strike Industries Sling Catch. They have begun selling it online. I plan on purchasing one. It looks like a comfortable way to carry a rifle when wearing a pack (or body armor).
I think they should also target the hunting industry with this product. My hunting rifle slings are all wide and thick to make carrying them comfortable. This gadget would allow me to use thin nylon tactical slings without them digging into my neck.