Under federal law they are not ... you just need to pay the man for the permission to own one.
(at least to be in compliance with an unconstitutional law)
A sbr can either be an sbr or a machine gun. If it is capable of more than one round per trigger pull it is a machine gun.
What are NFA Weapons?
There are two kinds of firearms under U.S. (federal) law, title 1 firearms and title 2. Title 1 firearms are long guns (rifles and shotguns), handguns, firearm frames or receivers, and most NFA weapons are also title 1 firearms. Title 2 weapons are NFA weapons. Title 2 of the 1968 Gun Control Act is the National Firearms Act (26 USC sec. 5801 et seq.), hence NFA. Title 1 is generally called the Gun Control Act, (18 USC sec. 921 et seq.). NFA weapons are sometimes called class 3 weapons, because a class 3 SOT (see below) is needed to deal in NFA weapons.
These weapons may also be further regulated by states or localities, and while these weapons can be legally owned under federal law, some states and localities further regulate ownership or prohibit it (see below). The NFA Branch of ATF administers the National Firearms Registration and Transfer Record, which necessarily encompasses most of the NFA regulation.
NFA weapons are: machine guns, sound suppressors (a.k.a. silencers), short barreled shotguns, short barreled rifles, destructive devices and "any other weapons". A machine gun is any gun that can fire more than one shot with a single pull of the trigger, or a receiver of a machine gun, or a combination of parts for assembling a machine gun, or a part or set of parts for converting a gun into a machine gun. A silencer is any device for muffling the gunshot of a portable firearm, or any part exclusively designed or intended for such a device (see discussion below). A short barreled shotgun is any shotgun (shoulder fired, smooth bore) with a barrel of less than 18" or an overall length of less than 26", or any weapon made from a shotgun falling into the same length parameters. A short barreled rifle is a rifle (shoulder fired, rifled bore) with a barrel length of less than 16", or an overall length of less than 26", or any weapon made from a rifle falling into the same length parameters (like a pistol made from a rifle). In measuring barrel length you do it from the closed breech to the muzzle, see 27 CFR sec. 179.11. To measure overall length do so along, "the distance between the extreme ends of the weapon measured along a line parallel to the center line of the bore." 27 CFR sec. 179.11. On a folding stock weapon you measure with the stock extended, provided the stock is not readily detachable, and the weapon is meant to be fired from the shoulder. A destructive device (DD) is a explosive, incendiary or poison gas weapon, or any firearm with a bore over 1/2", with exceptions for sporting shotguns, among other things (see discussion below). Any other weapons (AOW's) are a number of things; smooth bore pistols, any pistol with more than one grip,(but see below) gadget type guns (cane gun, pen gun) and shoulder fired weapons with both rifled and smooth bore barrels between 12" and 18", that must be manually reloaded (see discussion below). These definitions are simplified, to see if a specific gun is a title 1 or 2 firearm one needs to refer to the specific definition under the statute(s), and possibly consult with the Technology Branch of ATF. There is also case law on the issue of whether a specific item falls into one of these categories.
Owning or making an NFA weapon
