I've propbably posted about this maybe twenty-seventy times
But anyhoo, Although the 28 day nonsense is absolutely a cooling off period, or a cool-your-jets-before-you-decide-to-go-slaughter-everyone-because-you're-mad-and-4-weeks-is-a good-random-length-of-time-to-have-a-think. . . . . . ..even if you have firearms (some states) you're obviously going to need a different firearm....than one in your safe. .
Because its a concept based on absolute STUPIDITY; it doesnt actually refer to cooling-off, in fact the NFA states:
"....to enable appropriate checks to be made on licensees in order to ascertain whether circumstances have occurred since the issuing of the original licence which would render the licensee unsuitable to possess the firearm or which would render the licensee ineligible for that type of firearm."
In other words its a check that you're still licensed! If you couldnt possess firearms you would have long been visited by the boys in blue...sorry, BLACK....
Either way, it achieves the aim of stifling ownership, and giving the police and anti-crowd a warm feeling that they've 'made a difference'.. placing another little hurdle...totally unnecessary.