I sincerely doubt gun slings were originally developed as a shooting aid, because too many early guns (including early rifles) weren't accurate enough to benefit from a shooting sling. Instead, slings became dual-purpose as rifles became more accurate.
Another little piece of evidence is the sling swivel studs on many European shotguns, which I find very handy, especially when turkey hunting, where a shotgun is carried far more than shot. Which is exactly why many turkey shotguns have sling swivel studs.
As somebody named O'Connor once pointed out, rifles are relatively clumsy burdens when carried, because their comformation is primarily for being aimed and shot. A sling is a simple and natural way to make them less clumsy burdens between shots.
O'Connor's wife didn't like slings, instead carrying her rifles like Steelhead carries his, one reason detachable slings are so popular: If you don't like slings, for whatever reason, then you can take 'em off. Or if you have ambivalent feelings about slings, you can put 'em on or take 'em off.
“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.” John Steinbeck
