Can't speak to slug guns in specific. However, my first "classic" double was a Sauer 16 with swivels. I found a European sling to fit. (Expect that would not be an issue with an RBL; European shotgun swivels are typically narrower than American slings, which are almost exclusively made for rifles. In Europe, it's pretty common to have a swivel-mounted sling on a double.) Found it quite useful--for taking pictures without putting the gun on the ground, trudging back to the truck with a limit of birds, etc.
