Hawkin type guns have the replaceable barrels over ramrod and one screw into butt of stock. Got that on some of my T/C's. Otherwise, the leather strap around barrel and pouch for butt plate style of sling. However, these get used rarely! Gun is usually cradled under arm. Only truely good for long walks when not expecting to see game!
I bought a real nice home made one at the gun show at the Wash,Co.Gun Show a couple weeks back.A real nice older couple have a shop in Beaver Falls called Shablesky Muzzle Loading Supplies.The Mrs.handles the m/loading stuff and makes the slings herself and other leather products.It was only $15.00.Her number is 724-843-5925.
Not sure where I got it but I use one like these with my Lyman Great Plains. It is suede, laces onto the butt stock and loop over the barrel with care not to get into my site picture. I don't use it all the time or even every year. If I'm out for a long walk in the state gamelands, it is nice to carry it slung.