I've read perhaps 15 books , mostly the kind dictated to an author by some relative of ,or second generation person who had actual knowledge of life and times from 1750 to 1820 living in Appalachia . The folks providing the original information seem to seldom mention construction details of individual guns. Generally , slings are one item almost never mentioned. There being almost no way to know what is historically correct , we can only apply modern common sense to use of slings on rifles. My guess is most day to day information is taken for granted and valuable print isn't to be wasted on what was commonly known details, back in the day.
The information says, horses were heavily used on the frontier back in the day. I cant imagine a person riding through a woods on a horse w/o the barrel catching on some brush or limb and pulling the rider off onto the ground. There is no mention of scabbards in use either. A rifle slung over the shoulder frees up the hands as well.
Examining old rifles in collections , occasionally , diagonal holes in forearms are evidence an upper sling swivel once was applied. Lower swivel fixtures were part of the trigger guard , or a metal sling button screwed into the lower butt stock european style. Lehigh Valley , Pa. rifles have a boss on the front of the trigger guard for the lower sling swivel. The boss is a dead giveaway in identification of where such rifles were made. Military muskets universally employed a sling so why wouldn't some of the custom made rifles used here for war have had slings. There's just little information on slings in early use.
Most folks ordering a deer hunting rifle these days want a leather or cloth sling . As a gunstocker , all I can go with , are the little bits of actual "eyes on" evidence of how slings were applied , that being , if muzzle up , the muzzle not protruding much above the top of the head , rifle slung on the shoulder, butt down. If the lock's hammer is caught on something to pull the hammer out of the safe notch and the rifle errantly fires , the charge goes safely upward. In 50 yrs. m/l hunting , as yet , I've never seen this happen.
If anyone has anything to add to this , please inform us.....oldwood