I first would like to say "DON'T DO IT!!, but I know that there are always some people who have to push the envelope. First off, let's look at the official definition of what a full auto arm is. The B.A.T.F. defines this as a firearm that can fire more than one round with a single pull of the trigger. Interestingly, this definition would not include some mechanical arms like the Gatling gun. It also makes legal certain trigger devices such as the BMF activator, Tri Burst, and a number of trigger cranking devices. Many people are surprised to learn that fully auto weapons, though highly regulated, are legal for civilian ownership. This involves a $200 transfer fee which buys you a stamp, and the legal ability to do business with a class 3 dealer. The guns are very expensive, and there is a limited number of them due to a law that was pushed through congress in 1986 by those watchdogs of virtue and safety, the democrats. Essentially the law states that no new fully auto arms may enter the civilian market after a certain date in 1986. There are ways around this law (salesman samples and the like, also previously owned police weapons), but the additional regulations, as always, add expense and difficulty. The $200 stamp also imposes the requirement to be finger printed and photographed, and the application must be signed by the head of your local law enforcement or by a judge.
    As far as the mechanics of full auto are concerned, it is a very simple operation, and most of the weapons are not that complicated. The simplest full auto weapons are actually less complex than their semi auto counterparts. In a semi auto, the hammer is held back by a small notch, called a sear. When the trigger is pulled, the sear moves, allowing the spring loaded hammer, or striker to slip off, and ignite the primer in the cartridge. The bolt, or slide is forced back by the recoil or by a gas system. As the bolt moves back, it latches on to the base of the spent cartridge with a small spring loaded hook (the extractor). This ejects the spent case, leaving the chamber empty. As the bolt moves forward again, it strips the uppermost cartridge from the spring loaded magazine and pushes it into the chamber. These operations are identical for both fully and semi auto weapons. Where they function differently is in the way the hammer or firing pin is controlled by the trigger/sear. A semi automatic weapon will have a device called a disconnector linked to the trigger. The disconnector is a sort of a second sear, which catches the hammer or striker if the trigger is still back. The disconnector will stay engaged until the trigger is released, at which time it will release, and the hammer will fall back on the regular sear. You can sometimes hear this taking place, particularly on the M-16/AR-15. If you dry fire an AR-15, and keep the trigger back, listen as you let up on the trigger. You will hear a slight click as the trigger comes foreword. This is the sound of the hammer being released from the disconnector, and being caught by the primary sear. A full auto weapon will usually also have a disconnector, but this will only be engaged if the gun is set to fire in semi auto mode. So it would seem that the easiest way to convert an arm to full auto would be to remove or disable the disconnector, but this will generally not work, and is not the way most fully auto arms are designed. The exception to this is the open bolt gun. Every semi auto civilian weapon that I can think of fires from a closed bolt. This simply means that the bolt is all the way forward, against the chamber, and the cartridge is fully inserted in the chamber with the bolt at rest against it's base before the hammer is released. In an open bolt weapon the cartridge stays in the magazine until the weapon is fired, where upon the bolt strips the cartridge from the magazine, chambering, and firing the cartridge at the same time. Many open bolt weapons have no hammer, or firing pin, but use a point milled into the face of the bolt to ignite the primer, using the inertia of the bolt. When the trigger is pulled it releases the bolt to chamber and fire the cartridge. This method of operation is often referred to as "slam fire". In this type of weapon, removing or disabling the disconnecctor will indeed convert the weapon to full auto. This is why there are virtually no open bolt civilian weapons. Another reason for the scarcity of this type of weapon in civilian hands is its vastly inferior accuracy, and its some what inferior reliability. The only real reason to design a weapon with an open bolt is for full auto firing. Examples of open bolt weapons are the MAC-10/11, certain models of the Uzi, and the M-1 version of the Thompson (not the excellent M-1927).
    Most military weapons, and virtually all civilian weapons fire from a closed bolt. These weapons are superior in every way to their open bolt counter parts except in the matter of simplicity. In semi auto mode, the operation is as described above. In fully auto mode, the disconnector is bypassed, and a new disconnector, called an auto sear is engaged. When the trigger is pulled, the weapon fires the first round in the usual way. When the trigger is released, the primary sear catches the hammer, as it would in semi auto mode. Where things differ is when the trigger is held back. In this case neither the primary sear nor the disconnector catch the hammer. If there were no auto sear, the bolt and hammer would both go forward, and a round would be chambered, but it would almost certainly not be fired. The reason for this is the inertia of the hammer and bolt. The bolt is much heavier than the hammer, and much slower moving. The hammer is relatively light, but moves very quickly when the trigger is pulled. When hammer and bolt move together, the bolt slows the hammer down so that it has not the force to ignite a primer. What is needed is a way to keep the hammer back until the bolt has gone fully forward, and chambered the cartridge, at which time the hammer would be released. This is what the auto sear does. Using the M-16 as an example, the fully auto hammer has a hook on the end. This hook engages the auto sear, which is set in a pin so it can pivot. When the bolt moves back, it pushes the hammer back. When the bolt moves forward again, the hammer is allowed to move forward also, and will continue to travel with the bolt until it is caught by a sear. If the gun is set to full auto, then it will be the auto sear which catches it. After the hammer is caught, the bolt continues foreword, chambering a cartridge. The M-16 bolt carrier is a tubular piece of steel, with piece cut out along the bottom for about half of its' length. The auto sear is a small L-shaped part, which sticks up into the hollowed out portion of the bolt carrier. When the bolt carrier comes all the way foreword, it reaches the end of the cut out portion, and the tubular edge trips (pushes on) the upper part of the auto sear, causing it to pivot. This pivot releases the hammer and allows it to strike the firing pin which, in turn, ignites the primer. Every closed bolt, fully automatic weapon I can think of uses some variation of this method. That is to say, they all have a means of holding back the hammer or striker, until the bolt is fully foreword, and the cartridge is seated.
    Much has been made of the ease with which certain weapons may be converted to full auto, but this is simply untrue. Again using the AR-15/M-16 rifle as an example, here is what needs to be done. First, you must replace much of the internal firing mechanism. This would include the hammer, the trigger, the disconnector, and the selector switch, along with their assorted springs. You would also need a new M-16 bolt carrier, as the AR-15 one is milled back almost its' full length. All of these parts may be bought legally, as they are only parts, but they are not cheap. You can spend a couple of hundred dollars on these parts, depending on the quality you desire, and on whether or not you buy them  individually or in parts kits (so called "repair kits"). The bolt carrier in particular can cost $60-$80. After you have collected these parts, you must get an auto sear. Auto sears are restricted parts, and they must be numbered and registered just like a full auto weapon. This includes the requirement for the transfer fee, and fingerprinting. They are also expensive, and the auto sear will likely cost you more than all of the other parts put together. If you are going to purchase a legal auto sear, you may as well just purchase an entire weapon, as the requirements are the same. Assuming you have all of these parts, you must then mill out a portion of the receiver, as AR-15 receivers are thicker walled than their M-16 counterparts. It is necessary to drill a very precisely positioned hole in the receiver for the pivot pin on which the auto sear is mounted.
    As all of these parts are small, and are operated at considerable speed with considerable force, the margins for error in the drilling and milling operations are very small indeed. You would need a very well equipped machine shop and a pretty significant level of skill, as you are in essence rebuilding the mechanism of the gun. The point I am trying to make here is that any one who could convert a semi auto to a full auto would also have the skills and equipment to construct a full auto from scratch. On top of all of this, the largest cost might be that you have spent all of this time and money on a weapon that will land you in prison, if you are caught with it. A man who puts a gun to someone's head and threatens to kill him would be in less trouble, if he is even caught, than you will be if you are caught even owning an illegal class 3 weapon. This is not fair, and I do not believe it is constitutional, but this is the way things are. I own no class 3 weapons, but who knows. If I ever do decide I want one I will have to scrape up the money, wait several months for delivery, and have to move to a jurisdiction where the appropriate officials will sign the papers
    If you feel that this is unjust, and improper, there are four ways to deal with the situation. The first, is to do nothing, decide that there is no point, and that the process is more time, trouble, and money than it is worth. In truth, this was the intention of fdr, and the rest of the democratic bums who drafted this legislation. In point of fact, there is a certain logic to this, though far too many people passively submit to this trampling of their rights. These hurdles are more trouble than it is worth for a weapon with such a limited application for the civilian. The second way of dealing with this, is to abide by the spirit of the law, and to fulfill all of the requirements, and labor under these restrictions. The third method is to disobey the law, and illegally buy, or convert a weapon which fires in fully automatic mode. This route is also, taken by far too many people, though by a considerably smaller number than those who passively submit. The fourth, and best path to follow, is to make an effort to get the law changed. This too, may be more effort than it is worth for such a limited weapon; but it is something to think about. Individual rights are small things, taken separately, and are easily argued away in exchange for convenience, security, or prosperity; but there is a cumulative effect.
    No single blow of the hammer, or cut of the chisel creates a sculpture; but each strike changes the face until the work is complete, and what was once a certain thing is transformed into a completely different thing. Fully auto arms were initially taxed, then restricted. Finally, there was a ban placed on any fully auto arm manufactured after September of 1986. In the meantime, restrictions have been placed on so called assault rifles, certain pistols, various other types of firearms, and even certain magazine capacities, and bullet styles. Such restrictions would have been unthinkable, and unacceptable years ago. Today they are merely small steps for which precedents have already been set. Who can say what the next step might be?