On your journey to getting your Federal Firearms License, (FFL), you'll unduly run across a wide variety of acronyms, confusing definitions, and just plain legal legalese. At first glance, the task of getting an FFL may seem insurmountable, but it's not. Do not let this be a deterrent. As cryptic as these regulations seem, once you familiarize yourself with the terminology and basic ideas behind them, getting a license becomes much easier. With that in mind, let's talk about the NFA and the types of weapons covered by it.
When dealing with firearms, you'll often run across the term NFA firearms or NFA weapons. It stands for the National Firearms Act and is a law that was enacted in 1934. Not only does this law call for the mandatory registration of all Title II weapons, it requires that an excise tax be paid on the manufacture and sale or transfer of these weapons. Another important facet of this law is that it requires that any transfer of title II weapons across state lines is to be reported to the Department of Justice.
So what are Title II weapons you ask? Well, in the eyes of the government, there are two types of weapons – Title I and Title II. Title I weapons are primarily rifles, shotguns, and handguns. Title II weapons are machine guns, silencers, short barreled shotguns, short barreled rifles, and the any other weapon category, (AOW). One common misconception regarding Title II weapons is that they're often called class 3 weapons. This is wrong, there is no class 3 weapon. class 3 refer to the class 3 SOT, a special class of license that's needed to become a dealer of NFA firearms.
Now that you know a little bit about the NFA, and the two types of weapons, let's look a little closer at the Title II weapons that are covered by the NFA.
Machine gun – This is any gun with the ability to discharge more than one cartridge from a single trigger pull. Also included within this category are the parts that make up a machine gun.
Short barreled shotgun, (SBS) – This includes any smooth bore shotgun with a barrel length of less than 18 "or an overall length of less than 26"
Short barreled rifle, (SBR) – Much like the short barreled shotgun, the short barreled rifle is any rifled bore firearm that has an overall length of less than 26 ", or an overall barrel length of less than 16".
Silencers – These include any devices or parts that are designed to silence, muffle, or disguise the sound of any portable firearm.
Destructive Device, (DD) – This category includes two separate classes. The first covers grenades or explosive devices, poison gas weapons, or bombs and incendiary devices. The second class covers large bore, non-sporting firearms. By definition anything that's not used for sporting with a bore over 1/2 "falls under this class.
Any other Weapons (AOW) – This category is for weapons and parts that does not fit the other categories. It covers any shoulder fired weapon with a barrel length between 12 "-18". These can be either smooth or rifled bore. It also covers smooth bore pistols, cane guns, and pen guns.
This is only a general overview and should in no way be considered as definitive. If you're in doubt or need specific answers, check directly with the Bureau of Alcohol, Tobacco, Firearms, and Explosives. Their technology branch can definitively answer any of your questions.