This is what a bump fire stock is and how it works.

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.

https://www.youtube.com/watch?v=0MjR_fTHHpk

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

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.

  1. 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.
  2. The firearm is shouldered, and the shooter pulls forward with their free hand on the handguard of the firearm.
  3. 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.
  4. When there is enough pressure on the trigger it releases and the weapon fires.
  5. Recoil from the weapon, thanks to Newton’s Second Law, forces the firearm backward in the stock.
  6. The backward motion of the firearm moves the trigger back away from the stationary trigger finger, allowing it to reset for the next round.
  7. 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.

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71 COMMENTS

  1. I’ve always thought that bumpfire stocks were just for messing around with primarily. I’ve never shot one but I’ve read they’re hard to control and not the most accurate things in the world.

    Having shot countless rounds full auto in .mil, a semi auto suits me just fine for my AR, but it would be fun to have one for shiggles

    • Perhaps accuracy is not as big of a concern when you’re firing into a crowd & not aiming at a specific target, or alternatively if you are attempting to lay down supressive fire. I wouldn’t know. I’ve never done either, and I’ve never fired full auto or with a bumpfire.

      • Nobody could put down those long strings of fire with those ‘little’ 30 round standard mags that they are showing in the pictures of his room. And there’s not nearly enough brass on the floor.

        • “Nobody could put down those long strings of fire with those ‘little’ 30 round standard mags that they are showing in the pictures of his room.”

          I’m not sure I understand–are you suggesting that he wasn’t the one firing on the crowd?

      • Yea I agree with the crowd thing, but from where the bodies fell it looked to be somewhat concentrated. I don’t know if 400 yards would be too far to get that concentration of fire with a bump fire or not. Maybe someone who has shot one can shed some light.

        I know with a 240 it would be like shooting fish in a barrel thoigh

        • 240 woulda shredded the people / vehicles / etc. (and left bigger brass) but I know what (I think) you’re thinking regarding a belt-fed weapon a 249 woulda done the job quickly (but maybe too quickly [quicker than it actually happened]) the pauses in fire seemed like a rookie box/belt change lag. Maybe he was just reloading the 60 round mag? Something is weird. The pic looks like a certain book depository in Dallas. VERY tidy crime scene there.

          AGAIN, no way that pic of his bi-podded weapon was strewn with enough brass to be even close to half, and that weapon didn’t have a bump-fire stock on it.

        • Joe R.

          He had two firing positions that he was going between. That could explain why there isn’t more brass.

        • With all due respect, that won’t stop mass shootings. Considering, that only 1/40 mass shootings since Columbine have used a bump stock. You don’t even need a rifle, all you need are pistols. See Virginia Tech.

        • You are WRONG! From an engineering standpoint (I know you are not an engineer), the firearm functions just as it should…one round expended with one pull of the trigger…the “bump fire stock uses “inertia” (look it up) to SIMULATE full-auto fire–nothing more…
          The ATF has no jurisdiction over firearm accessories…

  2. I own several legal NFA Firearms with the giggle switch. I’ve also shot an AR with a bump fire stock.

    Either way shooting more than a few rounds quickly becomes more or less uncontrollable as far as accuracy. Some more than others of course. The bump fire stock was not easy to operate correctly and would probably take a lot of practice and I would think accuracy would suffer tremendously.

    In this particular case I don’t think the shooter cared about accuracy so it probably didn’t matter either way.

  3. I own several legal NFA Firearms with the giggle switch. I’ve also shot an AR with a bump fire stock.

    Either way shooting more than a few rounds quickly becomes more or less uncontrollable as far as accuracy. Some more than others of course. The bump fire stock was not easy to operate correctly and would probably take a lot of practice and I would think accuracy would suffer tremendously.

    In this particular case I don’t think the shooter cared about accuracy so it probably didn’t matter either way.

  4. “According to previous reports, the shooter in Las Vegas had two firearms on tripods with scopes”

    Count with me- one leg plus two – BIPOD

    • There are lots of tripods for shooting out there and they are very common in the sniping and precision shooting world.

      to TTAG: This commenting system still sucks

    • Bipod or tripod, if he used support he didn’t use a bump stock. They don’t function if the rifle can’t reciprocate in the stock, and they can’t reciprocate in the stock if they’re touching the ground (either directly or through a support device.)

      Bottom line, if a report comes out that he used a bump stock to fire off a bipod (or tripod,) it’s a lie. That’s no more physically possible than “he just pulled the trigger at 500 to 800 RPM.”

      • You are misinformed. How sad. Check youtube for “bumpfire bipod” or “bump SAW” and you will see this is not hard to do.

      • You are aware, I presume, that bipods have springs that allow you to preload them, in order for the recoil impulse to be more directional (straight backwards into your shoulder) rather than wasting some of the energy in muzzle rise.

        This preload is also a way to provide the forward impulse that’s necessary for the bumpfire stock to work.

        (Also, it didn’t occur to you to search youtube for “Slidefire Bipod” to see the 4150 videos available on the subject, none of which relate to the Route 91 Festival attack?)

        Personally, while I wouldn’t be in favor of banning bumpfire stocks, I think that they’re silly and pointless and would never spend the money on one myself.

  5. OK, do we let them ban them? Because it’s certainly going to be discussed.

    I’m ok with throwing them that bone, because it’s more of a toy than a useful tool.

    • Camel’s nose (or perhaps the jackass’s butt) under the tent flap. Or give the bastards an inch.

    • Bull freeking sh_t. Push back 350%. That’s what they do.

      We want the federal government to subsidize weapons and bump-fire systems purchases for all citizens, and we want there to permanently be tax-free transfers of firearms.

      The SHARE Act (or better) is a no brainer, and the evil POS (D) needs to be taken to task for their (apolitical – yeah right he was an evil (D) or else the evil POS (D) would be claiming that the apolitical shooter was a Republican and likely Trump supporter).

      • Hey Anon, we blame you for your Murphy. Fix your broken sh_t. Don’t worry about the SHARE Act, or “banning” any fing thing.

    • I don’t want to tell Americans what they can do, but I do have a word of caution. The gun control effort in the 90s in Canada started with the bone of mag capacity and licensing. The antigunners didn’t see that as enough and pretty soon .25 and .32 pistols were banned. I don’t see them caring much for a compromise.

    • I’m fine with throwing bumpfire shit under the bus, especially if we can get something out of it. They’re just stupid toys anyway.

        • Fudds may be ill-informed and annoying, but at least they’re not digging holes then pushing the rest of us in. Simulated full auto is some white trash bullshit anyone with a brain would have known was going to bring problems.

      • Not another inch. Grabbers should lose their illusion that they can get their way and ban another “toy” every time some a-hole uses it in mass murder.

  6. This is just BS speculation right now, as so much is, but it’s my BS speculation.

    When they start matching up brass to rifles, my guess is that he’ll have used most of the rifles at least once. he may have only used a couple mainly, but I’m thinking his intent was to spread the killing over as wide an assortment of arms as possible. Sick S.O.B.

  7. Here’s my question… bumpfire stocks are woefully inadequate. From the range the shooter was aiming from, a bumpfire stock would be the LAST thing I’d reach for. I realize he had a large mass of targets, but still.

    • Compare it to cannons firing grapeshot during the civil war. Sling enough lead into a crowd of people and you’re bound to hit someone. Indiscriminate murder was the sole objective.

    • I don’t understand why anyone’s debating range or accuracy. 59 dead bodies is the response, and it ends the debate.

      The killer didn’t randomly selected his tools or targets. In doing so he landed on a very rare set of circumstances where a bump fire device becomes incredibly effective. The fact that they’re so inaccurate and otherwise useless for any purpose other than chewing up ammo and wowing friends makes all that much harder to defend them.

      If they haven’t been legislated against by 2018 I’ll be stunned.

  8. The accuracy/understanding of firearms and how they are used/function in regards to tripod/bipod or any bump fire devise is circumspect when it come to the fake news/maine stream media.

  9. According to the ATF a “machine gun” is any firearm which fires more than one cartridge with each pull of the trigger.

    Close but no cigar.

    “Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot without manual reloading, by a single function of the trigger.”

    This is why binary triggers are legal, at least until the ATF decides otherwise. The pull and the return, each firing a shot, consist of two functions.

    These days a proper legal machine gun is as rare as a living doctor with a Porsche 911.

    Also close but no cigar. The Porsche 911 is now and always has been The Dentist Killer.

    • They’ve come a long way in taming that rear engine snap oversteer. My old 911T only had like 100hp so I never really got into trouble with it.

      The real dentist slayers were the 1980’s 930 Turbos.

  10. don’t see how they will be able to ban bump fire stocks as long as pre 1986 machine guns are legal NFA. most they will accomplish is making them also NFA.

    • Declaring bump-fire stocks are a machine-gun conversion would actually be a win for gun rights. Every single receiver they’ve touched is now in the registry as a proper machine-gun that can have a third hole added to it.

      • Wouldn’t that be an illegal unregistered or unapproved modification and then eligible for jail time? No thanks. Let’s hope not

    • I suspect they’ll be added to NFA with a grace period to get existing ones registered. I don’t see a buy back scheme passing, the Dems won’t want all the ones already in circulation being unregulated, and outright confiscation is unlikely (and unrealistic) at this point.

      All that being said, if you paid for yours with a credit card or had it shipped to your house, don’t assume that information will be private for long.

  13. Well that’s that… I called it.

    You can expect these to be added to NFA in short order at the very least. Don’t count on enough Republican support at any level of government to stop it. At best the bump fire stock will be a sacrificial lamb for Donnie’s tax plan.

  15. “These days a proper legal machine gun is as rare as a living dentist with a Porsche 911.”

    What does this mean? A dentist who owns a Porsche 911 is not at all a rare occurrence.

    • It’s a very dated and somewhat tasteless joke, referring to a time in the 80’s when certain 911’s, popular with medical professionals having more money than driving talent, were twitchy in power and handling leading to often fatal crashes.

  16. The bump-fire stock exists BECAUSE THERE ARE LAWS AGAINST THE LAWFUL TRANSFER OF POST 1986 FULL-AUTO WEAPONS IN THE U.S.

    LAWFUL PEOPLE WILL FIND A WAY TO LAWFULLY AND OVERTLY CIRCUMNAVIGATE SUCH LAWS, AND THOSE ENFORCING THE LAWS ARE BOUND TO INFORM THEM OF WHERE AND WHEN SUCH CIRCUMNAVIGATION IS SUCCESSFUL.

    UNLAWFUL PEOPLE DON’T EVEN PAY LIP-SERVICE TO THE LAW. So the ATF&E (THE “ENFORCEMENT” AGENCY [AND ALL OTHER LEOs]) CANNOT
    AND
    DO
    NOT
    PROTECT YOU FROM SUCH THINGS AND SUCH PERSONS WHO MIGHT WISH (AT ANY TIME THEY SO CHOOSE) TO DO YOU HARM.

    Arm yourselves accordingly.

    • Why do they exist? Because the Dems illegally voted in a ban on making new full auto in 1986. Nullify that unconstitutional law and there won’t be a market for bumpfire stocks.

  19. In youtube they compare bump fire to fully automatic AR and there patterns and rate was close to the automatic. In saying that, length of fire was a bit long, but with all the arms there in his room you would only have t drop one and pick up another? We all know to long a rate of fire causes barrel problems. There is a lot of unanswered questions here. You can bump fire all semiautos. The bump fire stock allows a better control of your field of fire, instead of firing from hip. Also machine guns are not illegal to own or shoot, just need the proper license and tax stamps. Just saying. As far as not enough brass, lets assume that they might have brushed a bunch of brass out of the way to not step on the brass itself? With his money why didn’t he just buy .308 auto for the length of shot he had and for the effect?? Lots or unanswered questions.

  20. Our shooting club does not allow bumpfires. No range should allow them. The problem is that a shooter could lose momentary control of the weapon due to recoil (particularly if it is a AK47) and put a round out of the range and into the surrounding community. Not very good publicity.

  21. So, this begs the question, are old Winchester 1897 pump shotguns or old model 12 shotguns considered machine guns?. Why, because you can keep the trigger pulled and keep racking the slide and it will continue firing with only one pull of the trigger (being held down). See how this topic can become just ridiculous.

    If you have a lot of experience with AR-15’s you don’t even need the bump fire stock, this can be done with holding it against your hip and it will rock and roll.

    It just amazes me how ignorant the general population is about firearms.

    • Slam fire shotguns are not considered machine guns. Reason? You have to manually reload them between shots.
      “Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot WITHOUT MANUAL RELOADING, by a single function of the trigger.”

  22. Hey Scoutino, I know this. I was attempting to point out how the leftists will use anything to distort facts. All day I have been watching the news media spouting off on how a bump stock converts an AR to fully automatic which obviously is not true. It is despicable and evil how they will use any tragedy to promote their gun grabbing agenda.

  23. Tell ya what: Write a bill that moves bump stocks to NFA AND removes suppressors from NFA at the same time.

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