Not to drag up a few month old thread, but I know kennesaw cannon makes 26.5 mm adapters for .22lr and .410/.45lc. They come with the batfe approval letter. It's feasible to get a 37mm to 26.5 mm adapter, then drop the .410/.45 adapter into that and your cool flare launcher becomes something more fun. Maybe someone with more nfa knowledge can comment on that.
Note: the barrels in those adapters are rifled.