[cymbeline]

“They have a rifled barrel to handle ...”

You’d think they’d would make a version with a smooth barrel.

[Eagles6]

Then it may be too short under the law.

I don't know.

You can get a .410 barrel shorter than 18" for a Thompson Contender though.

Better, I think, to stick with a rifle/carbine or a shotgun plus a handgun.