Or bees nest or whatever they're calling it now.

I have seen them in rifled, and non rifled, with a sprinkling of .22 shells and with a dozen.

In reality though, what are the odds of one of these rounds turning sideways or coming at some weird angle BEFORE the bullets exit the tube?

I'd hate to goof up a perfectly good 40mm tube by shooting a blast of 22 shells through it, although it does seem pretty neat.

Same goes with the other sub cal adapters like 12 and 20 gauge, I doubt they could have the chance to spread to 40mm before exiting the barrel but..maybe they can?

I had a couple 12ga adapters a while back.  They were somewhat short (similar in size to an M781 chalk round) and were bronze colored.
These particular ones weren't good, I was getting light primer strikes (shell too far from breechface?).
I successfully shot a couple WWB birdshot shells through one of them and I found a mark in the barrel that looked like one of the shot pellets hit the inside of the barrel a couple inches from the muzzle.  I don't think it damaged the barrel, but I stopped using the 12ga adapters.

No experience with the hornets nest.

YMMV

I've had good luck fixing this issue on 26.5 and 37mm adapters by putting an o-ring under the rim.