A discussion with a buddy of mine over my son's desire to have a black powder conversion revolver to play with led him to say he could just load black into a 5.56mm case and his AR would probably work twice before needing extensive cleaning!
Which, of course, leads me to wonder if anyone actually ever did that? Of course black wouldn't be able to have enough power in that little case to operate the system, and it would be probably the worst of unholy messes to clean in those innards...but just for grins and giggles anyone know anyone who actually DID that? No, not planning on trying!

Don't know but someone tried it with 9mm handgun, maybe a Glock.  They had to use the heavier 147 grain bullets for the action to work.

I wouldn't.

How does black powder react to that much bottleneck?

Interesting, but nobody tried it in 223 - makes sense as it probably wouldn't have enough ooomph to get the bullet down the bore!