They used taxation as part of the mechanism of regulating machine guns, but they weren’t regulated for the reason of taxation. That is, the goal of the NFA wasn’t to increase federal revenue; the goal of the NFA was to reduce the availability of machine guns, and it was effective in that goal.But they've been heavily regulated for the reason of taxation.
In 1930, a Tommygun cost $225, which is equivalent of $3220 today, and you could buy them at a hardware store.
The required tax stamp after 1934 was $200, which was almost as much as the gun itself, and not many people had the equivalent of $6000 to plunk down on one during the depression. So the government essentially made them cost prohibitive to the masses.
Not to give them any ideas, but I'm surprised the greedy pigs in DC haven't raised the cost of an NFA stamp to $3000 to keep pace with inflation.
For the individual in question, his biggest problem isn’t that he might be assessed a fine for not paying the $200 for the tax stamp. His biggest problem is that he is now facing 10 years in Club Fed.