I think what you are missing here, and what I have not seen anyone moot in this controversy, is the obvious: what business has the federal government arming 130,000 personnel? Law enforcement personnel. Clearly, if ammunition purchases by federal agencies become significant in what you correctly describe as an unimaginably huge national economy, then federal agencies simply have far too many armed personnel!
Stated more rigorously, name for us the enumerated power that permits arming federal law enforcement personnel against domestic citizens at all. I can't put my finger on one. I can, however, argue that arming federal troops with military weapons (while under the NFA unconstitutionally denying exactly the same arms to citizens), and using US Army personnel to train these federal troops in military tactics, to be employed against domestic citizens, violates at least the spirit of the Posse Comitatus Act.
That's how the government enforces the laws. You will pay your taxes, put out your cigarette, put on your seatbelt and shut up.
And if you claim that ain't freedom, you're un-American.