They didn't need a reason to pass the 1934 national firearms act.
Machine guns were banned "because criminals used them" but what they failed to mention is that most if not all of the machine guns used were stolen from the US military and law enforcement, not bought in a legal transaction.
Silencers were banned because someone might poach to feed their family, that's literally the justification, so really they need no justification. Oh no can't have people feeding their families.
They tried to get handguns on the NFA too but that didn't fly.