Unless you are going to do the paperwork and pay the tax, dont do it.
"you can make a 12 ga double barreled pistol, but you d better not be seen trapsing around with it. way too much legal nonsense around short barreled guns and classification and beurocracy and such its likely to cause all kinds of trouble if seen by the wrong people."
This is terrible advice. If you are caught with an unregisterd NFA item, you will be jailed for 10 years, and be forced to pay a $250,000 fine.