I've seen a few people mount 37mm flare launchers to various ARs. All of the barrel mounted ones slipped around when the rifle was fired more than a few magazines. I would get a rail mount if I had to have one.
If you just want to shoot smoke and flares and pyrotechnics, no problem with a 37mm. If you decide later you want to shoot "knee knockers" or rubber rounds or gas or beehive rounds or something you can always Form 1 your 37mm as a DD. I think Spikes was selling 37mm registered as a DD on their site.