Here' a picture of the only rifle sling I can find at the moment.
A plain leather strap, sewn on the ends to length. Nothing much to it.
Wood and iron last 200-300 years, no problem. Leather, on the other hand...
The Poser rifle that is in "The Great Guns" has a simple greenish woven linen strap. Attached to the swivel in front with a brass button (kinda like the way they use a "Chicago screw" now), and I think put on the rear with the stocks sling button. No hardware at all.
I have a photo of a rifle sling that belonged to some nobleman. It is about 3" wide, black leather, with two smaller ends sewn on so it can be attached to the sling swivel and button. It has large metal letters attached to the face of the sling standing for person's name and title. It's in one of my many auction catalogs, and I haven't found it yet... If I remember correctly, it's dated around 1800 or so. If you have "The Collectors' Illustrated Encyclopedia of The American Revolution" it's constructed like the embroidered sling shown in that book. I wonder about the date of that sling...
In the period images I have showing slings on rifles, I always see just a simple strap. Nothing fancy. Feel free to browse through this album:
www.photobucket.com/albums/v326/Fatdutchman/Jaeger