The ATF Form 1 and Form 4 for manufacture or transfer of an NFA weapon have a section for why you need the firearm.
In the comments, what would you put (or have you put) as a reason to need to own an NFA weapon? Funny, serious, and combination are welcome.
I like ‘Ahh, Zombies!!!’
http://www.saysuncle.com/archives/2006/07/27/fun_with_form_1/
Perhaps, “To prevent a tyrannical form of government form taking over”
“My neighbor’s dog suggested it, and I had to agree is sounded like fun.”
I put “For all Leagle purposes” they bought it.