I suppose I should've been more precise, due to the piles of online folks who will take things out of context, put words in my mouth, etc.
Shotguns would be NFA if they weren't exempted for sporting purposes. Airguns don't count as NFA, and "spud guns", well, they seem to vary from letter to letter, state to state.
If you have a tube that is going to be used to fire a projectile, that is smooth bore, that is over .50", that is a firearm
in any legal sense (using a burning propellant to project an object over distance), you have an NFA item in the large bore destructive device category. Doesn't really matter if you're "only" using model rocket motors (which is how actual AT rocket launchers essentially work...), doesn't really matter if it's "only" a toy, in a pure legal sense it's within the NFA.
(There, is that more precise?)
Frankly, if you don't believe me, you can ask the NFA tech branch, or you can read the law, or you can look at existing examples. To be NFA compliant, tubes are either registered and sold as DDs, or they are rendered compliant by being "demilled" to whatever specs the BATFE is currently happy with.
By the by, if you go by the thinking that if there's no explosive projectile it's not NFA, why in the world are 40mm grenade launchers required to be registered? Why are mortars required to be registered?