A 37mm (flare) launcher is not an NFA item. However , if you possess this launcher with any "ammunition" that includes any kind of ballistic device, it then becomes a Destructive Device and is subject to NFA restrictions.
So if you want to get any kind of dummy rounds with rubber bullets or anything like that, you need to register it and pay the $200 stamp.
I am about 85% certain of this. So please research it further.