...and indeed don't even have a sling or carrying strap on the rifle when hunting DG ( African)...
Yes, if hunting DG then the rifle needs to be in your hands, not hanging off your shoulder, and in that case the loose sling can get caught up in brush etc
I would say the splinter foreends of double rifles is where the barrel mounted sling idea originated - just not enough room to screw a sling swivel into the stock and have clearance with the hand. My CZ550 Magnums have the stock mounted sling base but there is plenty of room and I've never been "bitten" by the sling mount on my 458. Those CZ550 Magnums modified in the US as the Custom Classics (?) in 404J, 500J and 505Gibbs have the sling mounts on the barrel - likely a good idea on the 500s but probably just for looks on the 404.
