Could a gunsmith legally build...?

Each item listed falls under different statutes. This is a question for BATF. You may build any firearm not banned by the NFA, or the state where your business is located. You cannot build any ammunition that can be deemed to be armor piercing and explosive projectiles are prohibited in most circumstances. It is obvious you can build a shoulder fired rocket, which was done recently on national television by a gunsmith in Louisiana. However the projectile was non-explosive and the weapon was for reinactments of WW2 scenarios. To deal with automatic weapons you will need a Class 3 license. Contact BATF, or read federal statutes for more precise information. Good luck.