There is a "sticky" thread at the top of this forum to answer that kind of question for you (here:
http://www.thehighroad.org/showthread.php?t=507319), but here's the brief run down:
The National Firearms Act of 1934 defined certain types of firearms. Under "Title I" of that Act are rifles, shotguns, handguns, etc. that can be bought and sold by any dealer and usually traded around among private citizens, depending on state laws.
"Title II" of that Act defined another set of firearms -- machine guns, silencers, short-barreled rifles, short-barreled shotguns, destructive devices (large bore firearms as well as explosive ordinance and chemical weapons), and "Any Other Weapons" (sort of a catch-all group for pen-guns, cane-guns, wallet-guns, smooth-bore handguns, handguns with a forward vertical grip, etc.).
"Title II" weapons are more restricted than "Title I" guns. To purchase or make one you have to register it with the BATFE and pay a $200 tax. When you get your canceled tax stamp back (usually months later) you can take possession of (or make) your new weapon or silencer. You cannot give, lend, buy, sell, etc. that item to any other person without them filing the same paperwork. You also cannot transport most of them into another state without filing a form to do so. Many states also have special prohibitions against certain NFA Title II firearms so you'll need to make sure whatever you want is legal in your state.
A Special Occupational Tax Class 03 dealer is a Federal Firearms License-holder dealer who has paid extra fees to be allowed to SELL NFA Title II items.
A Special Occupational Tax Class 02 manufacturer is also an FFL who is licensed to MAKE such things.
Each type of SOT has specific restrictions on what they can and can't do, but both licenses are for people in the business of making or selling such things -- not for private individuals who just want to own them.
Owning any of those devices is not terribly difficult, but each one requires its own tax stamp. You aren't "licensed" to own them, each one you want to own is simply registered to you with the BATFE.
