What Are NFA Weapons?
Title I guns are long guns, handguns, and firearm frames or receivers. Title II weapons are NFA weapons (which include machine guns, short barreled rifles and short barreled shotguns, silencers, destructive devices, and AOW ("Any Other Weapons"). The National Firearms Act (NFA) was originally enacted in 1934, and was subsequently given teeth by the enactment of the Gun Control Act of 1968. The Gun Control Act was enacted quickly in response to the recent assassinations of JFK, RFK, and MLK. Title I of the Act is generally known as the Gun Control Act (18 U.S.C. 921, et seq.). Title II of the Act is generally known as the National Firearms Act (as recodified - located at 26 U.S.C. 5801-5872). The Gun Control Act of 1968 (GCA), primarily restricts the transfers of typical firearms between persons. The National Firearms Act of 1934 (NFA) imposes a taxation and registration scheme on certain rare or unusual firearms (imposing the $200 registration tax). Both Acts have been amended and now prohibit transfers to individuals of questionable character (among other characteristics listed in 18 U.S.C. 922), and ban machine guns manufactured after 1986. This information and more can be found here.
The ATF has extensive information on their website about the kind of weapons that must be registered in the National Firearm Registration and Transfer Record. Some of those weapons include: Machine guns, the frames or receivers of machine guns, any combination of parts designed and intended for use in converting weapons into machine guns, any part designed and intended solely and exclusively for converting a weapon into a machine gun, any combination of parts from which a machine gun can be assembled if the parts are in the possession or under the control of a person, silencers and any part designed and intended for fabricating a silencer, short-barreled rifles, short-barreled shotguns, destructive devices, "any other weapon." This information and more can be found here.
All parties to a NFA gun trust must keep themselves apprised of the most recent regulations promulgated by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (BATFE). NFA gun trusts will undergo new requirements imposed by ATF 41F in June, 2016 that requires a passport style photograph and fingerprints be submitted along with a copy of the gun trust and ATF application for all persons involved in the trust who are considered "responsible persons." More can be read about ATF 41F here. If you are thinking about getting an NFA gun trust then now is the time to get it. The regulations go into effect in June, 2016. Even if you don't have your Title II weapons picked out yet, it may be better to go ahead and register your NFA gun trust before any possible changes to be issued by the BATFE.