I'd do some research on using a BBL insert in a 37mm though, think about this:
Seems that legally it would be no different than adding a cylinder conversion kit to a cap and ball pistol: An individual manufacturing a conventional firearm out of a non-firearm, which is legal to do. There's a distinction between "firing something other than a 37mm flare out of a flare gun" and "manufacturing a small bore cartridge firearm out of a non-firearm" which is what a barrel insert does.
It would need to be a rifled bbl if for a rifle/pistol cartridge and you can likely make & use it without any papers at all.
For a shotgun cartridge adapter (smooth bore) the adapter itself would need to be dealt with as a SBS on a Form 1, but not as a Destructive Device.
The flare launcher itself becomes a DD if you decide to load up 37mm buckshot cartridges.
A letter from BATFE would be a bonus to ensure no adverse ambiguity.