Do you know if anyone still makes and sells .410 adaptors for 12-bore guns?
SHOTGUN BORE ADAPTORS
I can’t find one on the UK market at the moment, although there are plenty available to the American shooter.
I remember testing one against a pattern plate about 20 years ago and I was amazed at the quality of the pattern, considering that the adaptor tube was only a few inches long.
As to the lack of popularity these days, I am sure that the fact that .410 cartridges are generally around £30 per 1,000 more expensive than 12s has a lot to do with it.
You might as well shoot a very light, or even subsonic, 12-bore than go to all the fiddle of an adaptor, then pay a premium for your ammunition.