I've got a question inspired by a dream I had last night. If someone were to load a firework into a firearm, would that result in the firearm being classified as a Destructive Device? For instance, lets say you took a standard, breach-loading 12 gauge shotgun (barrel diameter just over 18 mm), and you inserted into it an 18mm fireworks rocket, followed by a blank shotgun shell to ignite the rocket's propellant.
Would that cause the shotgun and/or firework to become classified as a Destructive Device under the National Firearms Act? If it does, what actions would the owner of the weapon need to do to avoid breaking federal law?