This is going to be a technical post, so bear with me. In order to understand what a “bump fire” device is and how it works, and how laws restricting them would be pointless, it is necessary to understand exactly how firearms work, how fully automatic firearms work, and how semi-automatic firearms work. Key technical vocabulary is highlighted.
Let’s start with the basics.
All modern firearms use cartridge (the “bullet” refers only to the projectile that is discharged upon firing). The cartridge holds together the propellant and projectile in a case (usually brass, sometimes lacquered steel), and has a small hole at the base (the back). Behind the hole sits a primer cup. The primer cup is filled with a very small amount of explosive material. To fire the cartridge, a firing pin hits and compresses this primer, and the explosive material in the cup ignites, sending a jet of flame into the cartridge case proper, igniting the propellant. The burning propellant releases gas (and heat), and the expanding gas pushes the projectile out of the case and down the barrel. Per Newton’s 3rd law, the force of the expanding gas and moving projectile also push backwards against the cartridge base, and if the gun holds together that force is transmitted backwards, through the firearm, to the shooter. This is called recoil. The gasses are also pushing out laterally against the case walls, which are contained by the barrel.
When a cartridge is fully supported on all sides, and from the back (by the bolt or breech block), it is said to be In Battery. Should the cartridge not be fully inserted into its barrel chamber, and / or not also supported at the breech (the opening at the back of the barrel) by the bolt or a breech block, then it is said to be Out Of Battery. An Out Of Battery cartridge discharge is very dangerous because the expanding gasses will travel by any available path, including through the relatively weak case walls, and these gasses can destroy a firearm in milliseconds (to say nothing of the shooter).
All firearms are designed to fire In Battery, whether they are single-shot, semi-automatic, or fully automatic (with some rare exceptions I can discuss in the comments).
The Firing Mechanism
The firing pin needs to hit the primer with some force. For simplicity I’ll use a rotating hammer, but inline coiled springs on strikers work in a similar fashion. The hammer rotates about an axis pin, and has notches on it near the base. These notches are designed to be grabbed by what is called a Sear, which is a pawl that prevents the hammer from rotating. Pulling on the trigger moves the sear out of these notches and allows the hammer to rotate. For safety’s sake, there are mechanisms in place to prevent this Sear from releasing unless the cartridge is In Battery.
There are two notches on the hammer. One is for when the hammer is held in a ready position for firing, one just for the sear to grab it while the action moves.
In a fully-automatic or semi-automatic firearm, some of the recoil energy, or gas energy (as tapped through a small port some ways down the barrel) is used, with a delaying mechanism that waits till the bullet is safely moving down the barrel (or out of the barrel), to move the bolt out of battery, extract the spent case, and rotate the hammer back, where it is grabbed by that Sear. Return springs save some of that energy to stop the bolt then shove it forward again, where it grabs a new cartridge and chambers it back into the barrel. When the bolt is safely back in battery, the mechanism allows the user to release the trigger, which lets the sear pull back slightly so that the hammer moves forward until it is grabbed again by the Sear on the firing notch. At this point the user has to pull the trigger to fire.
What makes a semi-automatic firearm differ from a fully-automatic firearm is this: a second Sear that grabs and holds the hammer back. This Auto-Sear is independent of the trigger, and usually grabs the hammer not at its base, but at its tip where there is another notch for it to catch. This sear entirely lets go of the hammer only when the bolt is back In Battery, so the hammer rotates again to strike the firing pin, etc. For this to work, the trigger Sear has to be disengaged enough so that it does not grab the firing notch on the hammer at all while the trigger is held back.
Safely Converting Semi-Auto to Full Auto (Not Legal since 1986).
To convert a semi-auto to full-auto safely and correctly requires the installation of that 2nd full-auto sear, along with a hammer, selector mechanism, and modified primary sear – in other words you have to swap out the guts of the gun as all legally-sold semi-auto firearms are made without the various notches and tabs that you would need to work together. For either an AR-15 or AK of any sort, this means you have to drill through the receiver to have a place to mount that pin, plus you have to obtain the requisite parts. Accurate drilling is a must if you want it all to work (truth be told, it’s not that hard as templates abound), and the parts are not difficult to find either.
But don’t try it!
A modified gun is easy to spot by anyone in the know as those extra holes and pins are quite visible.
Also, the BATFE operates on a doctrine of “Constructive Possession” — so while you can legally obtain and own all of the parts, if you own those parts and a gun that could be modified, they’ll prosecute you just the same as if you had actually modified the gun, nevermind that the law only covers guns actually converted. This doctrine is often critiqued as the “Pre-Crimes” prosecution, from the film Minority Report, and has seen a number of law-abiding gun owners (usually collectors) jailed for many years.
Lots of idiots (I do not use this term loosely) think they can make their gun full-auto by grinding away the normal safety sears in semi-auto firearms. The problems with this are threefold:
- By doing this you make the gun full-auto all the time.
- You run a great risk of Out-Of-Battery discharges that could destroy your gun and kill you.
- You may well have an out-of-control discharge because there is nothing to stop that hammer from incessant movement. Drop the gun? It could go off in and keep chain firing as it spins around on the ground, likely inducing fatal consequences to your legs. Pull the trigger? It may well keep firing until the magazine is empty.
Like I said, only idiots attempt this.
Federal Law prohibits the modification, through mechanical means, of any firearm in such a way that would allow it to fire more than one round without the operator having to remove his finger from the trigger. Mechanical devices include springs or spring-like objects (rubber bands, for instance). An early attempt at a run-around of laws against automatics was the use of “trigger toys,” which are springs inserted into the trigger guards of firearms to shove the trigger forward.
These devices use the unpreventable recoil of firearms to jostle the operator’s hand or arms, so that really the human body itself (your arms, hands, shoulders) is involved, along with the springs, as that Auto Sear.
An operator only lightly pressing the trigger would thereby allow a firearm to bounce or bump around and keep firing without removing his finger from the trigger, so the BATFE cracked down on these. They take practice to use well, but because they also require you to have a looser than normal grip and stance while firing, they take a lot of training to operate accurately, and can’t easily be fired from the shoulder.
And the uselessness of this crackdown has been amply demonstrated by guys strapping rubber bands (available at any office supply store) to their triggers, instead of installing springs. Still, if you try it, and a cop sees you trying it, you will go to jail.
So any mechanical device modification is forbidden, but what about something that isn’t really mechanical? A bump-stock is something different. These stocks come in two pieces — one fixed shaft, attached to the gun, and an outer shaft that can freely slide on the first shaft. The trigger is not modified in any way, external or internal. The operator simply grips the gun as normal, shoulders it as normal, but does not brace it too tightly against the shoulder. Instead, the operator uses his support arm (left arm) to shove the gun forward while his right arm pulls the gun backwards. When he fires, the recoil shoves the gun backwards as normal, but that left arm’s forward pressure returns the gun (and trigger) to the firing position where the trigger hits the shooter’s trigger finger. Thus the shooter’s finger is actually leaving the trigger between shots, while the left arm acts like a second spring.
Because this is not attaching a mechanical device to the gun, not a spring or catch or other machine, but the human body itself, the BATFE could not find any legal reason why it could ban this concept under the existing letter of the law.
The most famous of this is the Slide Fire system. I have used these, and they are easy to replicate. They are also entirely legal.
Nota bene: thanks to @johnwalker for catching some errors and goofs.