I don't like slings for upland bird hunting
I am not a fan of slings for pheasant hunting. They seem to get in the way when walking through tall cover. Also, I hunt with a field-bred English springer spaniel. Flushs come fast and with little warning except for a very "birdy" dog.
An experienced rooster isn't going to wait for you to unsling a shotgun to make a shot. Pheasant hunters need to carry their shotgun in a comfortable and safe ready position. I practice my ready position to the proper mount often in the preseason. When there is a flush, the mount is automatic, very quick and the same every time.
My 870 Wingmaster has sling studs, however. I change it over to a deer gun with a Hastings rifled barrel and scope. It is great to be able to sling the shotgun while dragging deer or climbing into a tree stand. This is ONLY done with a completely unloaded weapon and the slide and bolt open.
Sings are nice for waterfowling in waders too. You never have enough hands when waist deep in muck. Try untangling decoys with one hand hanging onto your favorite shotgun.
Bye