I never thought of 'chamber mates' in handgun calibers for a shotgun and found this intriguing, especially when the partially rifled adapters (Dina?) are $20. But then I asked myself - what for? Only the 10" - 18"rifled adapters above will offer anything like accuracy at more than 10 yards, so it's not like having a combi-gun or Bergstutz for overlappping seasons, as in, "hey while I'm out here hunting grouse, I may as well shoot a deer". While that may be feasible with the 10" or 18" adapters, for their price, another handi barrel is a very viable alternative. What about dangerous game and/or people. Agreed, 1 oz of #7.5 is not a useful load in your 20 ga under such circumstances. But again, "oh, that bear is taking an overly keen interest in me. I shall insert my .357 adapter, find my pistol ammo, load my gun and fire before he eats me". Not. Well what about "because we can"? Fair enough. That's a good enough reason for me.
But for the other reasons, wouldn't it be cheaper, easier and at least as effective to keep and carry a few Foster or Brenneke slugs "handi". One's theoretical dear becomes a realistic 40 yard possibility and both bears and bogeymen would have more to fear, methinks...