And/or the general design of slings fitted with multiple projectiles.
Is it a novel invention of the forums?

I don't doubt for a second that someone in history stuck more than one stone in a pouch and let it rip. Actual dedicated shotgun slings? I doubt it. Someone, somewhere might have made one (throughout almost all of human history I can see the technology existing to make one), but it would have been for fun.

Are there "real world" uses for a shotgun sling? I can really only see one and I don't know if it would really work but...

In an ambush situation, the prime reason I can see to attack with slings is the element of surprise and causing confusion. Perfect situation would be a choke point, maybe a gully or valley, and attacking from above, preferably from cover. By doubling up on ammo, you could potentially make your enemy think you have greater numbers than you do. Getting hit with 15 stones would be bad, 30 would be worse. Imagine if you have 30 slingers; now it's 60 stones. I know that the stones wouldn't have as much energy, but killing the enemy isn't everything. Getting bombarded with potentially hundreds of stones is not fun even if they aren'y getting through your shield and armour. It makes you want to leave and stop doing whatever you're doing. A few extra stones in the air could be pretty useful.

TOMBELAINE wrote on Jul 26^th, 2021 at 6:05am:

If one encounters larger flocks of smaller easily stunned animals?
Ducks and waterfowl might be stunned by a few small stones. And the same with tiny, sparrow birds using fingernail sized pellets. You wouldn't even have to use nicely shaped ammo because accuracy isn't your main goal.


Then again. There are alot of posts in the archives that explains how fitting several rocks into the pouch doesn't work as intended due to the spread of power.
TOMBELAINE wrote on Jul 26^th, 2021 at 6:05am:

Yeah, like that... 

I think in most cases if you want to throw more than one projectile with a sling, the best solution is to have more slingers That way you can throw "real" sling ammo and get the best of both worlds.

Curious Aardvark wrote on Jul 31^st, 2021 at 8:27am:

The most common critique I have encountered is the physics of shotgun slings. That is, investing the same energy in several small rocks instead of one large doesn't translate well. But I do FEEL like a flock of birds would be a viable target. But still. I need evidence.

My own critique is that I have hardly been able to find references to it outside of slinging.org.
These two links http://www.primitiveways.com/gourd_sling.html https://www.tapatalk.com/groups/paleoplanet69529/fiddler-39-s-new-gourd-sling-sh... show the same shotgun sling. But that's it. No more.

Searching "sling" already comes up with many things that aren't shepherd slings and "shotgun sling" is no different. But still. Are there any references to shotgun slings outside slinging.org besides those I posted.
If it really was used it must have been the single most rare and niche primitive weapon deviced (besides the kestros).

In my shotgun sling tests I used miniature ice balls.  They were approximately 10g each.  Here's the most recent video: https://www.youtube.com/watch?v=tcxhkPyXxd4.  The earlier ones I did are in the Description of the video.  I don't recall what the distance was.  Probably around 15 m.

I think using a shotgun sling could be effective if throwing into a flock of birds.  But I would never want to use such light-weight projectiles for hunting.  I think throwing more projectiles at once provides a better chance of getting a hit, but each individual projectile will need to be large enough to deliver a sufficiently damaging hit.