Ask Foghorn: What’s the Difference Between a Full Auto and Semi-Auto Only AR-15 Bolt Carrier?

OK so no one actually sent in an email for this one, but its a question I get asked all the time. And since I have nothing to do while I wait here for some video equipment to be dropped off in preparation for that bail out bag challenge segment I figured I would sit down and explain it quickly.

Semi-auto fire is easy, from a technical standpoint at least. Pull the trigger, the gun goes bang, everything resets, and the shooter pulls the trigger again for the next round. Even some forms of full auto fire is easy, such as the open bolt Thompson SMG or Browning BAR. But the trick is being able to have a select fire gun that fires from a closed bolt, giving the shooter increased accuracy (thanks to the closed bolt) and the option to go either semi or full auto (the select fire). Its also cleaner and less likely to jam, but that’s just an added bonus.

The issue with closed-bolt firearms such as the AR-15 is that there’s a locking mechanism on the bolt. When the bolt goes forward it rotates, locking the breech and sealing the chamber. This allows for increased pressure in the chamber and therefore higher velocity rounds as the bolt is closed longer and allows more powder to be burned safely. There are other options, such as the roller delayed blowback system in the StG-45 and MP5, but the rotating bolt system has been in use in combat rifles for the United States since the M1 Garand and it works pretty darn well.

This is an issue because it means the gun cannot fire until the bolt is fully locked in place, otherwise there could be some nasty consequences. So Stoner engineered a foolproof method of determining when the bolt was fully locked and the gun could fire again. Enter the full-auto lug.

If you notice, the profile of the AR-15 bolt carrier isn’t completely cylindrical. There’s a cutout on the bottom, and a large chunk where there is no material on the top or bottom. That’s by design, both cutting down on the weight of the bolt carrier and allowing the hammer to properly impact the rear of the firing pin when locked in place. But the lug on the bottom of the bolt plays an important role in full auto fire. When the bolt is finally locked firmly in place that lug will contact the sear release in the full auto mechanism and automatically fire the gun. It’s a pretty nifty solution to an interesting problem.

Semi-auto bolt carriers came into being because gun companies and legislators didn’t want the Average Joe to have anything remotely possibly able to be used to make something full auto, so they trimmed the lug off the bottom of the bolt carrier. A semi-auto only bolt carrier will not properly trip the sear release, rendering the firearm only capable of semi-auto fire.

The astute reader will notice that the average civilian AR-15 does not have this mythical “sear release” in the trigger group. For that reason, a full auto bolt carrier in a standard civilian AR-15 will not make the gun go full auto. You need some more parts, and generally the only people who have those parts don’t sell them to any Joe who asks.

So what’s the bottom line? For us lowly civilians, there’s no difference in function between a semi-auto and a full-auto bolt carrier. Either will make your gun go bang, and neither will let it go full auto unless you have the other required parts. It does however add some weight to your bolt carrier and strengthen it slightly, which is the reason that I like to go with FA bolt carriers. That, and my aspirations to one day be among the chosen few with a registered lower. Check your local laws though, as some states may restrict such things.

And if they do, move to Texas.

[Email your firearms-related questions to “Ask Foghorn” via guntruth@me.com. Click here to browse previous posts]

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About Nick Leghorn

Nick Leghorn is a gun nerd living and working in San Antonio, Texas. In his free time, he's a competition shooter (USPSA, 3-gun and NRA High Power), aspiring pilot, and enjoys mixing statistics and science with firearms. Now on sale: Getting Started with Firearms by yours truly!

26 Responses to Ask Foghorn: What’s the Difference Between a Full Auto and Semi-Auto Only AR-15 Bolt Carrier?

  1. avatarLouringe says:

    You also have the Federal issue of putting full-auto parts in a semi auto firearm

    • Some polititcian or lawyer will argue that you have “defaced” the firearm, and thus now have an illegal firearm.

    • avatarMoonshine7102 says:

      Not until you have the entire collection of parts which could be assembled into a working machinegun. As long as you are missing at least one part, you’re good.

  2. avatarSanchanim says:

    Nice article..
    Yeah maybe someday.. You know right after the hogs in the field out back sprout wings..

  3. avatarMike says:

    A M16/full-auto bolt carrier is OK in an AR under federal law:
    http://www.gandrtactical.com/images/archive/ATF%20M16%20Letter.pdf

    Also, the increased weight of a FA bolt carrier helps decrease bolt bounce.

    I had some store credit from at a local gun store here in the Houston area (known mainly for being built out of an old Circuit City). I asked the guy behind the counter if they had a FA bolt carrier or complete group in stock. He told me it was illegal, you needed a tax stamp, blah blah blah. Right below him was a sign advertising their house-brand AR’s, which featured (wait for it) FA bolt carriers.

  4. avatarMatt in FL says:

    What are the rules about possession of those other parts?

    I have a buddy that has an AR-15 with a FA bolt, for all the reasons listed here. The lower receiver is not, repeat not, machined to take a FA trigger group. He has a FA trigger group in a baggie, stored separately from all his firearms and firearm paraphernalia.

    I have two separate but related questions:
    Is his mere possession of that FA trigger group alone illegal?

    Would machining the lower receiver to accept the FA trigger group render the receiver illegal? If so, is it illegal in and of itself, or only for someone who is also in possession of a FA trigger group, or only after the trigger group is installed?

    I’m asking in regard to Federal laws; I’m not concerned about state/local laws for the purposes of this question.

    And yes, it really is a buddy, not me. I don’t own any AR variants yet.

    • avatarMoonshine7102 says:

      Possession is all that is required by Federal law. Someone in possession of a FA lower, a FA bolt carrier group, a FA trigger group and a FA sear is in trouble. Remove one of those pieces from possession, no trouble.

      • avatarSid says:

        Correct.

        Possession of ALL the necessary components is the trip in the law. If one piece is missing, it is not a functioning FA weapon by law. But if the owner possesses that component, then it is a FA weapon even if it is not assembled.

      • avatar16V says:

        That’s rather dicey. “Once a machine gun always a machine gun” is the guiding rubric.

        Possessing merely an unregistered auto-sear will definitely be used against you.

        • avatarAlphaGeek says:

          Holy zombie threads, Batman. I hope you guys realize you’re replying to a discussion from last summer…

    • avatarMike says:

      Owning full-auto fire control components is non illegal per se. However, I think the ATF’s position is that possession of of such parts simultaneous with the possession of an AR-15 constitutes “constructive possession” of an unregistered machine gun.

      With respect to the lower, merely drilling the third hole for the auto sear into the receiver constitutes making an unregistered machine gun (there’s more differences between an M16 and an AR-15 lower but that’s the most apparent one). Don’t do it.

      • avatarmatt says:

        the term youre looking for is Constructive Intent. And I cant think of anyone theyve ever gone after for only owning a FA bolt carrier. And if you want to get that paranoid, what could happen if you have a AR rifle and pistol? Or even just a AR rifle and a hacksaw in your house? Either way, it could be viewed that you were planning on making a SBR.

        • avatarMike says:

          I said nothing about a FA bolt carrier, I was referring to the fire control group, auto sear, etc. In fact I posted the link to the ATF letter that says that FA bolt carriers are kosher in an AR-15.

      • avatarDave says:

        You guys are forgetting about registered drop in auto sears…these are the “machine gun” part and can be installed in any low shelf lower. There is no “3rd hole” for an auto sear for these. You merely have to have a FA bolt carrier and a FA trigger group and drop in your sear.

    • avatarCowboyvet says:

      There is nothing wrong with having a full auto bolt on this weapon. Other weapon platforms the bolt could get you in trouble but that is mostly open bolt full autos. Having the fire control group is not illegal but if you also own the weapon they fit, then it is treated the same as full auto under the “Constructive Possession” ruling by the BATF. Milling any receiver to accept the full auto parts is also considered a full auto with or WITHOUT the parts. Bottom line is there is no reason to have a full auto trigger group unless you have a stamp for it or you are a NFA SOT. The gamble you play with if you even cross the line a little is 10 years federal time and $250,000 not to mention never being able to own a gun again. Good advice is to call your local BATF agent if you ever feel something might be an issue before you purchase it. They are usually great gun loving guys who will answer your questions right over the phone.

  5. avatargunfighter 2012 says:

    Misinformation. Disinformation. So happy together.

    or

    Shrodinger’s cat. Now available as a website.

    No offense to Mr. Foghorn intended.

  6. avatarMike Settles says:

    Federal law (and ATF policies) aside, you may have state law to contend with.

    As example, Washington State prohibits possession of machine guns, short-barreled rifles and short-barreled shotguns. They also prohibit the possession of ANY parts with which to make a semi-auto into a full-auto weapon. :-(

  7. avatarRoger says:

    Ok for those of you who do not own a m16 as I and other do here are the rules.

    First you can own a fa bolt with your ar.
    A m16 trigger group and a ar trigger are interchangeable.
    All FA m16 parts EXCEPT the bolt while owning a ar are considered MG’s in and of themselves.
    Lat even if you have the trigger group and the bolt you will not have a FA machine gun unless you have one of the following items. 1) A lightning link 2) A DIAS. 3) A auto sear and the hole for it and the area it needs to operate milled out ( unless you have a older ar with that area already machined out.

    Mg’s are very easy to get if your state allowed them. They ain’t cheap anymore Due to Regan’s 86 ban. A FA TRANSFERABLE ( meaning one a civilian can buy IE registered with the ATF prior to 86) m16 or converted one will run you anywhere form 8 to 20 grand. Most go in the 12 to 16 range. Prior to 86 you could take your ar send in a form 1 to the atf with your 200 dollar check wait a few weeks get the approved stamp drill a hole and put in your m16 lower parts and you had a legal mg for the price of the rifle the lower parts and the stamp about 800 bucks.

  8. avatarKillerCams101 says:

    Colt Civilian AR-15′s come with a full auto BCG! End of discussion!

  9. avatarSouthern Cross says:

    My SP1 had a FA bolt carrier. The A2′s bolt carrier had the bottom of the ring at the back machined away to prevent the FA trigger groups from functioning. As I remember, all SP1 and A1 types had the FA bolt carrier. The SP1′s carrier still had the forward assist notches, even though the SP1 receiver was a slick-side. This is my recollection from over 15 years ago.

  10. avatarDave says:

    Ok , I only use full auto BCG’s, to add more weight to the carrier. Nothing i do is illegal, i have made sure. I personally dont own any class 3 full auto stuff, i simply cant affod the gun tax stamp, or ammo to shoot. I simply use Young M16 BCG’s,. Ordered to my house, no special license needed. The M16 lowers will work with my uppers, so will anyone that pays 20k for a class 3 tax stamp and a pre 1986 lower. And the M16 carrier, i only use semi auto only lowers and all built as hobby guns now that im disabled, but i did work at a gun shop and range years ago. I should have gone into more detail before, sorry all, didnt want anyone thinking wrong of my posts, was only trying to help.

  11. avatarjames says:

    So to clarify – An upper with a full auto bolt group will be fine in say, a civilian Colt 6920? Any tweaking to the gas system needed?

    Thanks

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