There's that archaic BATFE guidance that you can't make a pistol from a rifle. If it starts as a rifle, it can only be a rifle. However, if you start with a pistol, it can be rifle later, and convert back to pistol. Back when pistols were starting to become the rage, that was in one of their "determination letters" on a specific design (Thompson Contender, I think...).
Other than that ^^^ interfering with anything, I don't know of anything else out there.
Might have to check the BATFE rules and guidance on smooth-bores...
*** I'm not a lawyer, and anything in this statement should not be construed as legal advice...