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As per the BATF website : http://www.atf.gov/firearms/faq/firearms-technology.html

(and their site changes weekly so this link may or may not work - you may or may not need to search the information out again on their site but as of this writing (11Feb2011) it reads as follows in regards to "Home built firearms"

What is a reduced diameter tube used for versus an original tube?

A big part of building a semiautomatic version of a full automatic firearm is making it so that it cannot be readily made to fire in full automatic mode. Of course, anybody CAN convert anything into full automatic given machining and welding skills and time, but the idea is to make it difficult, so that the home tinkerer cannot achieve it with hand tools and an hour or two of time. A reduced diameter receiver tube helps with this by not allowing a drop-in bolt from a full auto kit to fit. The bolt used for the build must be reduced in diameter to fit the undersized tube. So, if someone gets your home build and decides to convert it to full auto by simply getting a second kit and swapping some parts, namely the bolt, they will fail, as the bolt will not fit in your tube.


What is a blocking rail?

As mentioned in the above question, wherein a semiauto build  of a full auto gun cannot be readily convertible to full auto, some modifications will need to be made. Ideally, a reduced diameter tube would be used. However, this is not always the easiest option. This is where the blocking rail comes in. It is a round or square, or even flat but wide, key that is permanently welded inside a standard diameter tube, before it is final assembled, running along sufficient length to ensure that an original, unmodified bolt from a full auto kit cannot enter the receiver far enough to fire the cartridge in the chamber. Ideally, it is physically in the way of part of the full auto mechanism that was in the original design. The original bolt would have a slot cut along its length so that it will fit over the rail and move fully forward in the receiver. If you have the rail in the way, then the slot is cut through the important area, such as the original sear notch. This way, a full auto bolt cannot be made to work at all. As the receiver is assembled, the rail is too difficult to remove without extensive damage, rendering the gun useless. The rail is also helpful in that it physically blocks the full auto sear mechanism parts from reaching the bolt, provided its in the way as it should be. Another benefit of the blocking rail is that it can be used in almost any shape of receiver, it is not limited to tube receivers.

What is a disconnector, and why do I need one?

A disconnector breaks the firing cycle by interrupting the connection between the trigger and sear, i.e.., "disconnecting" the trigger. This keeps the gun from firing again and again, without the need to pull the trigger for each shot. In a system that would fail to fire again and again, it assures that the firing mechanism is reset, so that when the trigger is released, it will be ready to fire when the trigger is pulled again, eliminating the need to re-cock it between shots.

The BATFE defines full automatic as the discharging of more than one shot per pull of the trigger. Therefore, you need a disconnector to keep the gun from being an illegally made full automatic weapon.

What is a reweld?

A reweld is when the parts of a demilled receiver are placed back together and metal shims are used to recreate the original length of the receiver. Rewelds can be legal provided that provisions are in place to prevent the re-insertion of fully automatic parts prior to welding them back together. Rewelds are legal because the demilled parts are not actually parts but legally scrap metal. Make sure that any modifications to markings and provisions to prevent the installation of fully automatic firing parts are done BEFORE you reweld ANY previous firearm especially those that could be a select fire or fully automatic weapon.
 

What is demilled?

The ATF publishes guidance on how to properly destroy weapons in order for them to be no longer legally considered a firearm. These standards are considered the minimally acceptable method for legally destroying a firearm. Once demilled, weapons are often made into "parts kits" which may or may not come with the components that were "destroyed" in the demilling process. Any weapon that has not been demilled properly is subject to confiscation by the ATF at a minimum. It has happened before where companies have shipped parts kits that were not properly demilled.
 

What makes an smg build a pistol build?

The problem with most smgs are that they typically have barrels somewhere between 6" and 11" in length. This is coupled with usually having a stock. The stock adds greatly to the guns practical accuracy. The problem with a semiauto conversion build is that in order to qualify as a rifle, which it would be classified as since it has a stock, it must have a barrel with a minimum of 16" in length. The other problem is that as a rifle, it falls under 18 U.S.C. Sec 922 (r). This law defines the importability and assembling of "no sporting" rifles and shotguns. It  further goes to say that in order to qualify as a US made rifle, and avoid this law, it must have a minimum number of US made parts, which are from a specific parts list given in the law. The other parts of the gun do not count one way or the other. Many guns sold as kits are somewhat of oddities here in the US, and little to no US parts are available. Between attempting to find a 16" barrel, or permanently attaching something quite long to the barrel to reach 16", and finding or making US replacement parts, a kit can be difficult to complete and be legal. An alternative is to build the gun as a pistol. It already has a short barrel, and usually is relatively short overall. The smgs usually fire a pistol sized cartridge, so handling them without a stock isn't a big deal. The best part is that 18 U.S.C. Sec 922 (r) doesn't apply to pistols, so other than the parts required to convert the gun to semiauto, no US made parts are needed.
However, the stock needs to be left out, as does the mounting hardware. It cannot be readily possible to mount a/the stock to the weapon.
 

What is an "80% receiver"?

Legally, There is no such thing as a "80% receiver", in the eyes of the ATF, a component is either a receiver/frame or it's not a receiver/frame. Manufacturers either submit a sample to the ATF and they make a determination or they follow a previously approved non-receiver component specification. In the hobby an "80% receiver" is a partially completed receiver that may be transferred without serial numbers or the use of an Federal Firearms License (FFL) because it has not reached the point where it is legally a receiver. As soon as the component does reach the point where it becomes a receiver it must be transferred in accordance with federal law. This could be as simple as bending a flat piece of metal into shape or drilling a few holes. One should assume that any change to an "80% receiver" may make that component legally a receiver unless the ATF rules otherwise.


What is 922(r)?

922(r) is the section of law (part of the GCA) that prohibits the import or assembly of weapons that are "not suitable for sporting use" that have more than 10 imported parts. It is a violation of the law to assemble any weapon that could not be imported under the "sporting use" clause. The ATF takes a very liberal interpretation of the law, so that it would be advised that no one should build an weapon using 10 or more of the imported parts listed in 27 CFR 478.39 without a letter from the ATF stating the weapon meets the definition of "sporting use."

    * (1) Frames, receivers, receiver castings, forgings or stampings
    * (2) Barrels
    * (3) Barrel extensions
    * (4) Mounting blocks (trunnions)
    * (5) Muzzle attachments
    * (6) Bolts
    * (7) Bolt carriers
    * (8 ) Operating rods
    * (9) Gas pistons
    * (10) Trigger housings
    * (11) Triggers
    * (12) Hammers
    * (13) Sears
    * (14) Disconnectors
    * (15) Buttstocks
    * (16) Pistol grips
    * (17) Forearms, handguards
    * (18) Magazine bodies
    * (19) Followers
    * (20) Floorplates

922(r) compliant means that no more than 10 of these parts on a weapon are imported.


What is NFA?

The NFA is the National Firearms Act, a 1934 law which provided for the registration and taxing of certain weapons. This included Machine Guns, Short Barreled Rifles, Short Barreled Shotguns, Silencers and other weapons. (Basically everything EXCEPT handguns, rifles, and shotguns which have specific legal descriptions.) Any weapon subject to this act must be transferred using the appropriate ATF Form (a form 4 for non licensees, e.g. those without an FFL) and have the appropriate tax paid prior to transfer. It is illegal to transfer these weapons to another person without paying the tax and having an approved form.
 

What is GCA?

The GCA is the Gun Control Act, a 1968 law which created the licensing scheme that we operate under today. It also created a number of restrictions regarding the transfer of firearms interstate and 18 USC 922 covers all the prohibited acts under the GCA in general. The GCA also known as Chapter 44 of Title 18 has been amended and changed over the years. Some of the changes have included the FOPA (detailed below) and the expired assault weapons ban of 1994. If you are looking for a federal law regarding firearms, it is likely in Chapter 44 of Title 18. This includes 922(r)
 

What is FOPA?

The FOPA is the Firearm Owners Protection Act of 1986. In light of the abuses of the GCA by government entities, congress passed this law that was initially aimed at securing the rights gun owners and license holders. Like most laws, it was perverted during the process and in the middle of the night an amendment was added by the democrats which basically banned machine guns for all of us. Since the number of legally transferable machine guns is essentially fixed the prices have skyrocketed. It did, however, secure the right to travel anywhere in the country with a firearm without fear of prosecution. For instance, it is legal, to travel from Pennsylvania to Florida with your firearm even if you pass through Washington D.C. provided your firearm is secured in the manner prescribed by law. It also limited the ATF in the number of inspections that could be conducted on compliant licensees.  

Do I need serial numbers on a home built receiver?

No, you do not need to place any identifying marks on a home built receiver unless you wish to transfer that weapon to another person at a later date or register the weapon as an NFA firearm.

Can I build a machine gun?

No, you cannot. Exceptions exist, but if you are asking the question, more than likely you're not the exception.
 

Is it legal to own a machine gun?

Absolutely legal, you must find one that was produced prior to 1986 and be prepared to spend $200 for the transfer tax in addition to the price of the weapon (expect it to be roughly 10x the cost of a semi-auto version of the same weapon.) Other than the Form 4, there is no special license required on the federal level for ownership of a machine gun.
 

Can I build a silencer, short barreled rifle, or short barreled shotgun?

Yes, but you must fill out and have in your possession an approved Form 1 from the ATF before building, and you will pay $200 for the privilege.
 

Can I buy a silencer, short barreled rifle, or short barreled shotgun?

Yes, but you must fill out and have in your possession an approved Form 4 from the ATF before you can acquire the weapon, and you will pay $200 for the privilege.

What makes a gun F/A ?

First, F/A is an abbreviation for full auto, or full(y) automatic fire. The BATFE defines this as a firearm that will discharge more than one shot by a single pull of the trigger. Any more than one, it doesn't matter how many.
Basically, for the average person, full automatic fire is when the gun fires itself after the shooter initiates the firing, and stops when the shooter interrupts the shooting. By pulling the trigger, the gun fires, either because a round is in the firing chamber and the action is cocked and ready for release, or because, in an open bolt design, the bolt is cocked back, and releases to chamber and fire a round. In either case, after the round is fired, the action is operated. It opens and extracts the fired case, ejects it, and chambers a new round, or, in the case of an open bolt design, the bolt is ready at the open position to close once again, chambering and firing another round. In full automatic fire, instead of stopping after the first round is fired and ejected, the cycle continues, simply starting where it had originally started, firing again. It will continue this cycle, firing, extracting and ejecting, chambering and firing, until the trigger is released, the gun runs empty of rounds, or it malfunctions.
In semiautomatic fire, the cycle is the same, except it is automatically interrupted after each cycle. This is the job of the disconnector. Because of this, the trigger must be released, and then pulled again, for each shot to be discharged. In a closed bolt design, the firing itself is interrupted, so that a new round is chambered, but not fired. In an open bolt design, the cycle will be interrupted at the point where the bolt is held back, after ejection of the empty case, ready to close, chamber and fire the next round.
It might be realized here that the open bolt design is quite simple, requiring very few parts, as the bolt just moves back and forth, the firing pressure pushing it back, and the recoil spring pushing it forward. All it needs is a catch to keep the bolt back, and pulling the trigger releases it to go forward, starting the firing cycle. The BATFE has realized this also. It is a rather simple alteration to either deactivate or remove the disconnector, and the gun will be a fully automatic weapon. Therefore, most open bolt designs are considered to be fully automatic firearms to begin with. In most, if not just about all, instances, the deactivation or removal of the disconnector in a closed bolt design will render the gun a single shot, as it will not reset itself and be ready to fire with another pull of the trigger. Also, it will not continue to fire as a full automatic firearm.
The simplicity and inexpensiveness of manufacture leads to many of the sub-machineguns that are cut up and sold as kits being of the open bolt design. However, to be legal to make into semiautomatic firearms, they need to be converted into closed bolt designs.

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