Binary Trigger – Reviewing the Best Way To Legally Simulate Full Auto

Binary trigger installed and going loud on the range.

Have you ever wanted a machine gun but can’t afford the high cost? Well, Franklin Armory’s Binary Trigger simulates full auto. The important part to understand is that as of this writing*, this is 100% ATF approved and legal. The BFSIII trigger is Franklin Armory’s latest AR-15 trigger design.

Binary trigger mounted up

The Franklin Armory binary trigger in its natural habitat.

*This article was originally published in August of 2017; cover photo courtesy of Franklin Armory. 

Franklin Armory Binary Trigger

Pull and Release

The Binary Trigger works by taking advantage of an interpretation of what is considered “semi-auto”. According to the ATF, semi-auto is defined by a single manipulation of the trigger and a single round is fired. Most triggers are what are usually called “pull triggers.” You pull the trigger and that releases the hammer to hit the firing pin which fires the round. While less common, release triggers do exist. They perform in a similar fashion however they only release the hammer when the trigger is released. You see release triggers in competitive shotguns like in Skeet and Trap. The trigger finger uses fewer muscles to release the trigger than it does to pull it. So when a shotgun shooter is ready to shoot some clays, they preload the trigger by pulling it back. They aim and as soon as they release the trigger forward, the shotgun fires.

Binary trigger from Franklin Armory

Franklin Armory BFSIII installed

The ATF recognizes the release of the trigger as a separate manipulation. So a pull and release trigger is considered two independent manipulations of the trigger and therefore is considered semi-auto.

BFSIII Trigger

BFS III binary trigger

The BFSIII binary trigger.


Everything in the photo above is what you get from Franklin Armory. What is not in the photo is a stiffer buffer spring but you get one as well. The buffer spring is only used in case you experience any issues. The trigger comes with two sets of hammer and trigger springs. This is to help with trigger pull. The safety is ambidextrous with a short and long lever.

Franklin Armory binary trigger on the SIG MPX (ambi).

Franklin Armory and the SIG MPX.

The safety works like any ordinary AR-15 trigger. The safe and semi-auto positions are the same that you are accustomed to. It is only when the safety is flipped to the third position, typically the full-auto position does the BFSIII shoot in binary mode.

What is actually happening is an engagement of the sear and hammer. While you are in binary mode you pull and hold the trigger to release the hammer for the first shot. The round fires and the bolt cycles back to reset the hammer. The BFSIII trigger has a second sear engagement that holds the hammer back and when you let the trigger move forward, it releases the hammer again and fires the second round.

At first glance, the BFSIII trigger looks very similar to an AR-15 trigger. But there are some subtle differences.

binary trigger vs. AR15 trigger

The binary trigger compared to a traditional AR15 trigger


Below you can see the BFSIII on the left. The disconnector is actually two halves.

BFSIII vs AR15 trigger

Another look at the BFSIII vs an AR15 trigger.


Franklin Armory BFSIII Installation

The installation of the BFSIII is similar to installing a standard AR-15 trigger. There is one step that requires a little more attention than usual. When installing the BFSIII trigger pin you need to pay attention.  There is a small spring that runs along inside the disconnector. This spring is held in tension by the trigger pin. Without the pin, it encroaches into the trigger pinhole. The BFSIII trigger comes with a slave dummy pin installed in the trigger that keeps that spring in place. All you need to do is install the real trigger pin. Here are some tips to make it easier.

I used a Geissele Trigger Pin tool to help me install the trigger and trigger pin.


Geissele trigger pin tool

The tricky part is when you are driving pins out and in through the trigger pinhole.  Go slow. If you don’t push the slave pin or the punch out smoothly, the spring will shoot forward and get in your way. If you continue to push out the pin or punch you can pinch the small spring. Should that happen, you will need a small tool like a dental pick or a thin Allen wrench to help push the spring out of the way as you push the Geissele tool or trigger pin through.

The Geissele Tool helps by keeping the trigger assembly aligned with the lower receiver so you can install the trigger pin easier.  If you manage to mess up and pinch the small disconnector spring, you can easily replace it. The spring is actually a cut down AR15 detent spring – though depending on your comfort level making field-expedient mods, you might be better served contacting Franklin Armory for a replacement. Installation of the hammer is the same as any other AR15 hammer.

Echo Trigger

Fostech: the competition

Fostech makes a similar trigger system called the Echo Trigger. The major difference between the BFSIII trigger and the Echo trigger is that the Echo requires a proprietary bolt carrier group. The benefit to the proprietary bolt carrier group is that you cannot outrun the trigger. This is a common problem with the BFSIII. When you go too fast, the hammer does not reset properly and you have a malfunction.

Don’t let that dissuade you from the BFSIII though! One downside to the Echo Trigger is the proprietary bolt carrier group. This limits you to what guns you can use it in. Now Fostech has come out with their Echo ARii trigger and it no longer requires their proprietary bolt but it does require a standard M16 style bolt carrier group to interact with the ARii trigger lock. This still limits you to AR15’s that use a full auto bolt carrier group. The BFSIII trigger is compatible with guns that take AR-15 triggers. No special BCG required.

Another competitor to simulated full auto is the bump fire stocks like Slide Fire. They work by using the recoil of the gun to push the firearm back while you push the firearm forward. This results in the trigger being pushed into your finger and firing the gun. They work but require a certain finesse to get them to work reliably. The amount of forward pressure is crucial. Push forward to much and the recoil will not push the gun back to reset the trigger and hammer.  I tried using one on my .22lr rifle and AR15 but could not get it to cycle continually.

You could also look into the hand cranks that pull the trigger as you rotate the crank. These require a little finesse in installation to get the right sweet spot so that it pulls the trigger and releases it so that the hammer resets properly.


Now For Some Rapid Fire

Here is a video of the BFSIII in an AR lower with a Wolf A1 piston-driven upper. I used a Magpul D60 drum mag and a Surefire 100 round mag provided by Gun Mag Warehouse. Both filled with 60 rounds of 5.56×45. I shot the D60 suppressed and the Surefire unsuppressed.

A post shared by Nick Chen (@solscud007) onFeb 26, 2017 at 1:55pm PST


Since the BFSIII does not require a proprietary BCG, you can use your BFSIII installed lower with any compatible upper receiver.

Here is a video of an AR57 upper on a pistol lower with the BFSIII trigger installed.


A post shared by Nick Chen (@solscud007) onFeb 26, 2017 at 6:58pm PST


Shooting an AR-15 in Binary mode is fun. But what if you pull the trigger in binary mode but do not want to let the second round fire? You simply flip the safety back to semi-auto or safe.

M&P 15-22: going binary

The most fun is putting it in my M&P15-22. You do need to modify the lower a bit so the binary safety works. Don’t worry this modification will not compromise the safety of the gun. The M&P15-22 polymer lower has a safety lever stop that protrudes out the side. This physically stops the safety lever from moving past 90 degrees. You just need to shave this plastic bump off and the BFSIII safety will work.

Installing a binary trigger on the MP 15-22

Giving your M&P 15-22 the gift of a binary trigger can lead to a lot of fun and inexpensive range time.


A post shared by Nick Chen (@solscud007) onFeb 8, 2017 at 9:21am PST



A post shared by Nick Chen (@solscud007) onFeb 8, 2017 at 9:22am PST




While a .22LR rifle is fun in binary mode, I also installed the BFSIII trigger into an MPX. The MPX has a spotted record with aftermarket triggers but according to Franklin Armory, they modified and designed the BFSIII to work in the MPX.

A post shared by Nick Chen (@solscud007) onFeb 25, 2017 at 3:56pm PST



So how does the BFSIII compare to a legitimate machine gun? Here is an MPX by Parker Mountain Machine that was modified into a select-fire weapon.

A post shared by Nick Chen (@solscud007) onJun 12, 2017 at 6:42am PDT


Here is a comparison to a transferable Mac11 vs my MPX with BFSIII trigger.

A post shared by Nick Chen (@solscud007) onJun 10, 2017 at 5:39pm PDT


The BFSIII retails for $429.99 but is currently on sale for $386.99 at Franklin Armory. They are currently working on a trigger for the Bushmaster/Remington ACR and just released a binary trigger for the MP5. Thanks to Franklin Armory to providing the BFSIII for review.

Franklin Armory binary trigger

  • Josh Olive

    Why are bump stocks mentioned as an option when they’re federally illegal?

    • DH350nWo

      Because the original article was written in 2017.

  • Mark Young

    Look I LOVE Guns and own 1 shotgun or as I call it a Biden special Gee gods 2 22lr rifles 1 AR 15 or as I call my Obama special and a Browning 7mm remington magnum or as I call it my shoulder hickey 2 22LR pistols and 1 9mm 1911 .So I think I have the right to say WE DON’T need this in our sport it’s just another item that the Leftist will bring up on why their needs to be more Gun laws If you ever have shot a full automatic AR -16 I have and you would know why you can’t hold it steady it goes all or the place and it will empty a 20 round Magazine in about 2 seconds it’s a burp and 20 rounds are gone. Unless you nuts or rich as hell who can afford to shoot it ! And I don’t want to give the Anti Gun rights crazies any more ammo than they already have.Please lets use some common sense and not give the Left wingers anymore reason to push for more GUN Control or as I say it PEOPLE control.

    • DH350nWo

      First, the binary trigger is closer to the 3 Rd burst setting than to full auto. As demonstrated in the article you control the rate of fire with each trigger pull.

      Second, views such as yours have allowed the loony left to infringe on gun rights. If the gun enthusiast community would fight harder (not just take the NRA’s word) against these infringements this wouldn’t be a necessary conversation.

      Our rights under the 2nd determine our needs. Just because technology changes, our rights do not.

      • Mark G

        Don’t agree, sorry.

    • The Bearded Pretender

      Mark you are a fake and a Liar.
      It is obvious you have never fired on full auto and I doubt you own those guns.
      Basically I think you are a Moron and a Poser.

  • betterboy

    The beauty of an AR platform is the accuracy which you lose with this device. Although this binary trigger or bump stock is a waste to me I do support the rights of those who want them. If you believe the Las Vegas shooting was real then you have to admit a bump stock is a useless pos. From the beginning – 58 people killed and 500 wounded. None of the wounded died or the death count would have gone up – very suspect – unbelievable. I don’t believe it happened as it has been told. I would be nice is someone in the know would tell what they know.

    • Charles Valenzuela

      EVERYONE was “wounded”. 58 of those that were “wounded” died.

      • betterboy

        Hours after the shooting it was stated that 58 people were killed, but none of the 500 wounded died in the following 2 weeks or the death toll would have gone up. I wonder what the mathematical possibility of not even 1 wounded person dying. People who are shot and killed might have died from a wound such has their heads being blown to pieces, but who considers them wounded besides you?

        • ricotorpe

          Do you really think the typical AR bullet will have enough energy at 400 yards to result in “their heads being blown to pieces”? No. Just no. Funny that you mention the “mathematical possibility” but you don’t bother to even attempt to figure what it is. You just assume that it is very unlikely without doing any work.

          I have noticed that conspiracy theorists are intellectually lazy.

          • betterboy

            Try re-reading. I never stated that those shot in LV had their heads blown to pieces – it was a general statement that those who are killed in that way are not called wounded. You counter my statement about 500 wounded and not 1 of those dying with a lack of statistics. I was expecting you to use common sense and experience from other disasters to see the improbably of not even 1 out of 500 dying. There is a conspiracy theory that Trump was spied upon by the dems. The facts are being discovered that will change it from a theory. If you think a conspiracy theory is a bad thing then you have been brain washed by those with something to hide. It has now been confirmed that building 7 was pulled and could not have collapsed from the internal fires. A theory now proven.

          • ricotorpe

            Yes, “pulled” is jargon from the demolition industry. There are conspiracies. But when you have a selective evidence filter to make one out of nothing, that makes you a foolish conspiracy theorist.

          • betterboy

            Most people still believe that the “federal reserve” is a gov agency. It would seem foolish to many that it would be a private corporation. Those who believe in crazy things like adam and eve or Noah saving a pair of every animal exist in many forms – like believing that Obama has been honest and a natural born citizen.

          • ricotorpe

            The Fed isn’t a gov agency, but it isn’t a private corp., either. But hey, you go ahead and memorize garbage. While you are tilting at windmills, the real power will roll over you, and you won’t see it coming because you are too busy screeching about someone being born in Kenya.

    • Peaver Bogart

      I don’t believe a bump stock was used in the Las Vegas shootings. I owned two bump stocks and the rate of fire is much faster than what was recorded at the Las Vegas shooting. I believe they were shooting an automatic weapon.

      • betterboy

        The gun fire recording sounds phony to me – like a tape recording and not real time. A machine gun cycles with the same intervals. The recorded sound has some varying intervals which sounds like a bump stock to me, but again you have to believe all the ridiculous “facts” we’ve been told to believe the shooting was real and not a false flag.

        • Peaver Bogart

          I don’t believe the shooting was real. I believe it was a false flag. The bump stock is much faster than an M16 or select M4 I could empty a 30 round magazine in 15 seconds.

          • betterboy

            The only auto I’ve fired was a Thompson in Las Vegas. It was a bummer because the ammo was likely reloaded with a low charge and didn’t perform as expected, and I’ve never used a bump stock. I was not referring to the speed of fire, but to the evenness of the timing. I’ve watched a video of bump stock fire and the interval varied due to the human element involved with the trigger. Since you have experience I would like to know your opinion about that. The recorded fire in LV did not sound consistent as it would from a machine gun.

          • Peaver Bogart

            To me,the Las Vegas tape sounded like a M16 or select fire M4 it sounded too slow to be a slide fire or bump stock.
            If one practices using your thumb through the trigger hole on any semi auto rifle hooked into your belt loop you can make automatic fire. so, are they gonna outlaw belt loops or maybe thumbs. LOL

    • ricotorpe

      Full-auto is of little value with most types of firearms because it isn’t easily controllable, so accuracy suffers. Same with bump-stocks. But there is one situation when they are useful, and that is when there is a crowd of people.

      You can just spray bullets, and you have a good chance at hitting someone.

      We don’t know much about the ARs he used, particularly the barrel twist rates as well as the mass of the bullets. Even if he fired adequately-stabilized bullets, he was shooting at targets 400+ yards away. That’s a lot of energy loss, so and 11% proportion of deaths is not surprising. As for the “wounded” not dying, that is a lie. A little under half of the dead died in a hospital.

      But hey, if it makes you feel special to have the “inside scoop” go right ahead and be a moronic sucker. Maybe you can harass some of the people who were there by calling them up and accusing them of being “crisis actors” or similar nonsense.

      Better yet, lock yourself in a closet with an AR, and do the honorable thing.

      • betterboy

        I don’t like being called a moron sucker, especially by a know it all. The first day it was reported that 58 people were killed. If half died in the hospital it is still dead on the first day. The death count never increased after that, so none of the 500 wounded died on any of the following days. If you don’t think that is odd then maybe you don’t know as much as you think. It seems you already did the “honorable thing” before you started posting here.

        • ricotorpe

          If you “don’t like being called a moron sucker” then don’t be a moron sucker.

          • betterboy


          • ricotorpe

            Baaaaawwwww! 😭😭😭😭😭😭😭😭

  • Corky Affolter

    now that this big mouth writer has brought attention to this trigger,it won’t be long before it will be outlawed. thanks buddy you know the trolls read these articles to!!!!!!