This isn't so far-fetched as it seems, there is precedent.

For example, following the 21st Amendment and the repeal of prohibition, we had a herd of enforcement agents who had nothing to do. This is part of what led to the National Firearms Act of 1934 -- ostensibly a tax measure (anybody could have a machine gun, as long as you were willing to pay the $200 tax). How else to explain the odd linking of unrelated products under the BATF -- the "Bureau of Alcohol, Tobacco, and Firearms"?

Also interesting is that the law was upheld by the courts not because the government had the authority to regulate firearms, but because the tax was a revenue-generating measure, not a restriction or prohibition. The court conveniently ignored that the tax was sometimes more than 10 times the price of the item being taxed and therefore effectively a prohibition.