Red, back in the days when I'd go through a few cases of 00 a year doing demos, I could load 4 in an 870, traverse fire on 5 targets, loading the 5th round through the ejection port,(part of the qualifier) and get good hits all down the line in under 5 seconds. Got timed a mess of times. Probably still could now.
In a crisis, I doubt that there would be any real/practical advantage in speed having a semi, and maybe a disadvantage.
And there ain't no big red S on my chest either. You could do the same, I'm sure, with practice. Expertise beats the hell outa hardware anytime.
Back to slings. These days I'm a private citizen, and we're talking HD here. Unless I'm headed for a clay or trap course, an 870 wears a sling when I leave with it. But, it the philosophy of keeping things simple, the HD 870 wears none in the house. SWAT teams have different parameters and needs.
Do me a favor. Walk through your residence with your longarm of choice. Every corner, hallway or room,see just what that sling can catch on. Next, try it w/o the sling.
Real, empirical evidence beats the H*ll outa theory. I'd be interested to hear your results and compare them with mine...