As details begin to emerge from the Las Vegas mass shooting, more questions are being generated than answers. According to the Associated Press the gunman in that shooting had two bumpfire stocks in his possession, but there is still no confirmation that they were used. So, what are bump fire stocks? Do they actually turn a weapon into a machine gun as the Associated Press suggests?

Let’s start with the definition of a machine gun.

According to the ATF a “machine gun” is any firearm which fires more than one cartridge with each action of the trigger. So if you have a double barreled shotgun that fires both barrels with the pull of a single trigger, that’s a machine gun. Machine guns have always been tightly regulated in modern times thanks to the 1934 Gun Control Act which required registration and taxation for all legal machine guns. They were legally manufactured for civilians up until 1986, when a new law made it illegal to register any newly manufactured machine guns. These days a proper legal machine gun is as rare as a living dentist with a Porsche 911.

In civilian hands a machine gun is pretty much useless for doing anything functional. Too inaccurate to be used for hunting, illegal in just about every competition shooting sport, and of questionable benefit in home defense scenarios, there’s only one place where it makes sense: the shooting range. Having fired my share of machine guns I can say that while they might not be practical, they are definitely fun to shoot. The reason why machine guns are fun is the same reason why fireworks are popular: the bright flash from the muzzle, the loud noise, and the recoil felt by the shooter are all exciting and entertaining. Until you tally up your ammo cost at the end of the day, that is.

A proper machine gun costs in the range of $20,000 which prices it well out of the range for the average American, and the limited number in circulation means that even if every gun owner wanted one there simply aren’t enough to go around. For those who still want to enjoy the excitement of full auto fire without the up front investment there’s a $100 add-on device for your existing semi-auto guns that will simulate full auto fire.

Nick tests SlideFire stock on Mean Arms hybrid rifle and burns his thumb. #stupidnick #burn #gun #slidefire A post shared by The Truth About Guns (@guntruth) on Apr 16, 2015 at 1:35pm PDT

It’s called a “bump fire” stock, and here’s why it’s legal.

Like we discovered earlier, machine guns are defined as firearms which fire more than one cartridge with a single pull of the trigger. As long as you pull the trigger once for every round that leaves your gun you’re 100% legal. The issue here is how to do that as quickly as possible and simulate fully automatic fire.

There’s a technique known as “bump firing” a firearm which will do an okay job of pulling the trigger as quickly as possible. Back in the day it was accomplished by sliding your thumb through the trigger guard and looping it through your belt loop on your pants, but these days there are devices like the Bump Fire System (or this one by SlideFire) that allow people to do it much easier and more reliably. Here’s how it works.

In a normal firearm, the stock is rigid and doesn’t move. With a bump fire stock, the firearm has about an inch of travel where the firearm can move back and forth while the stock stays stationary. It also has a ledge to keep your trigger finger stationary while the gun moves. With that configuration in place the cycle can begin.

The shooter positions their finger on the ledge on the stock, which keeps the trigger finger stationary in reference to the stock and just in front of the trigger. The firearm is shouldered, and the shooter pulls forward with their free hand on the handguard of the firearm. Pulling the firearm forward moves the firearm forward in the stock (where it moves freely and independently), eventually allowing the trigger finger to make contact with the trigger. When there is enough pressure on the trigger it releases and the weapon fires. Recoil from the weapon, thanks to Newton’s Second Law, forces the firearm backward in the stock. The backward motion of the firearm moves the trigger back away from the stationary trigger finger, allowing it to reset for the next round. As this is happening the shooter is continuously applying forward pressure to the handguard, like a spring. This moves the firearm forward once again to repeat the steps and fire continuously.

This process is a little difficult to understand, but I think the Instagram video above is slightly helpful in illustrating how it all comes together. The key here is that the shooter themselves needs to act like a return spring, slowing down the rifle as it moves backwards and then moving it forwards once again to contact the trigger and fire. It takes practice to get right as too much pressure or too little pressure will cause malfunctions and will keep the gun from firing.

Some may have noticed a key requirement here: the stock MUST remain stationary while the FIREARM reciprocates.

According to previous reports, the shooter in Las Vegas had two firearms on tripods with scopes pointed at the crowd across the street. If these were the weapons used in the attack then it would be nearly impossible for a bumpfire stock to be used — a tripod is attached to the firearm, not the stock, and would not allow the firearm to reciprocate properly for the bumpfire action to work.

However, if those firearms were not used and instead the shooter was firing “offhand” or from a standing position with a different firearm than the ones on a tripod then it might be possible that the bumpfire stocks were used.

As always there’s too little data and too much speculation right now to make any definitive statements about the situation in Las Vegas. But hopefully this will give some insight into how bump fire stocks work and why they exist.