$200 adjusted for inflation would be $3925.The were probably too busy trying to live their lives, and survive the times. NFA was an example of political elitism in the extreme. $200 was not feasible for the common American at that time, so basically only criminals who didn't care about the law, the government, and the rich were unaffected. It was a financial barrier more than anything and the politicians that passed it should have been hung for perverting the constitution.
I would bet that there wasn't many at that time that could fork out that much money for a silly stamp.