The History of Drop-In Auto Sears

When Colt decided to sell a semiauto-only civilian version of its AR-15 rifle, it had to make a number of changes to the design of the lower receiver to prevent full-auto fire control parts from being used. These changes would make a semiauto AR receiver legally distinct form a full-auto one, and thus not a machine gun in the eyes of the law. Among the changes Colt made was to remove the auto sear, along with its pin and mounting holes.

Now, at this time (and still today) the possession of full-auto fire control parts was not regulated. Owning an unregistered machine gun receiver was illegal, but the internal parts were not machine guns by themselves. The DIAS, or drop-in auto sear, is a small device that was invented to duplicate the function of the original auto sear without needing a mounting pin. It could simply be “dropped in” to a semiauto AR receiver and if used in combination with some of the full auto fire control parts it would result in a fully automatic rifle.

When the DIAS was first invented, it fell into the category of other fire control parts – it was not regulated in any way. They were very simple and cheap to make, and were widely sold by mail order. It was illegal to put one into a semiauto rifle, but not illegal to own one. This changed in 1981 when ATF realized what the DIAS really was, and by administrative decree the DIAS was redefined as a machine gun by itself, thus illegal to own. However, examples manufactured before the ruling were grandfathered in – not that there was actually any way to prove when a DIAS had been made.

In 1986, the Firearm Owners’ Protection Act (best known for its Hughes Amendment which closed the machine gun registry) was passed. It did many things – some of them actually good – but it also legally redefined the DIAS as a machine gun in and of itself. This set the old ATF ruling as actual law instead of administrative whim, and also overrode the original grandfathering. Ever since, possession of an unregistered DIAS has been a crime.

However, until the Hughes Amendment closed the registry, it was perfectly legal to register a DIAS, thus making it a LEGAL machine gun. Many of these registered DIAS remain on the registry today, where their value has climbed to nearly that of an actual registered M16 by virtue of the legal paperwork. By weight, a registered DIAS today is worth ten times more than gold…


  1. Now I know the means to framing someone for terrorism. Just sneak a facsimile of a drop-in auto sear into someone’s house, call the ATF, and watch the mess unfold. Just kidding.

  2. A couple of years ago, an idiot at Knob Creek was selling “80%” M-16 drop in sears. Drill one hole for the roll pin and mount the flapper and spring. I told him that was illegal AF, and he just laughed. Some young guys were looking at them, and I told them they were worth ten years in jail. Did they listen?

  3. The auto-sear by itself is worthless. You need a full auto bolt carrier, full auto hammer, 3 position selector and the disconnector needs to be modified. Aside from the bolt carrier, attempting to buy those other parts will set-off alarm bells.

  4. The essential insanity of solving violence and mental health issues through banning artifacts is going to be coming more and more clear as time goes on, and more people get access to fabrication technology. What was once achievable only with esoteric knowledge and a full machine shop at your disposal is going to be accessible to anyone with an internet browser and a 3D printer, no matter what the people thinking themselves the “authorities” may think.

    My take on the matter has always been that banning weapons of any type in an attempt to control human misbehavior makes about as much sense as going back to homo habilis and trying to ban knowledge of how well rocks and bones work as clubs. The fact is, the idea will always be there; the only thing you can do is work on the “will to use” end of things, and hope for the best. Some humans will always resort to violence to get the things they want, and you just have to be prepared to deal with them as they manifest their mental deficiencies.

    I’m reminded of the fact that I once observed someone’s Remington semi-auto hunting rifle go full-auto on a range because of a broken trigger assembly. Nothing the owner did or intended, but the fact was, he was in constructive possession of an illegal “machine gun” from the time he knew it was broken until the time he got it fixed, which was a couple of months. Until I pointed out the illegality of the situation, he was perfectly content to keep his “broken” weapon in the gun safe… At the time I researched the details on that situation for him, attempting to keep his stupid ass out of trouble, the deal was that so long as you didn’t know the weapon was broken (per case law…) you were OK, but the minute you did know, you had to immediately take action to either have the weapon destroyed or have it fixed. If you kept it, knowing it was “broken” in that manner, then you were liable for ownership of an unregistered and illegal machine gun. I’m not sure if the case law has changed, or if the guy who I got that information from was accurately expressing things as they stood at that time, but that is my understanding. A malfunction that you immediately deal with? No big issue; if you knowingly keep a weapon known to consistently malfunction, you’re breaking the law.

    Of course, if there’s bad will on the part of the people doing the assessment, you may be screwed even if you don’t know about the malfunction condition in your weapon. That’s been reputed to have happened, but not having personal knowledge of the situation, I’m just going to leave that out there as a potential.

    Anyone trying to sell you one of these things (and, there are still ads out there for them…) is either trying to entrap you, or dumb as hell. They’re also counting you being an idiot.

    The existence of those Glock “switch plates” and their prevalence in the gang-adjacent community is another argument for the essential futility of trying to ban this stuff.

    And, honestly? I’m of the opinion that if I’m going to be shot at by some nutjob dirtbag with anything, I kinda think I’d prefer them to be shooting something on full-auto. Based on my past experience, the average idiot shooting something like an AR-15 on full-auto is going to pose rather more danger to any birds or aircraft flying overhead than they are anyone that they’re trying to shoot at deliberately…

    Mental illness usually doesn’t go with careful planning and preparation. Occasionally, yeah, it does show up, but… In general? Not a major reason to worry.

    One of these days, people are going to recognize that you can’t make laws against things, and that the only way to effectively deal with these things by way of law is by working on the nutjob end of it all, not the weapons. Dude doesn’t have access to a weapon? He’ll make one; most of the really bad arson events or vehicles running into crowds are an excellent example of improvised weapons you really can’t do much about.

    Hell, if anything, you could probably make a good argument that the prevalence of weapons makes for reduced body counts on these occasions; if a killer has a gun, he’s constrained by the characteristics of it all. If he’s using a five-gallon can of gasoline or other flammable…? It’s pretty indiscriminate and potentially exponentially more lethal for more people. Not to mention, I think I’d rather be shot than set on fire… That’s just personal preference, though.

    Ya can’t fix mental illness without addressing it directly. Banning everything that someone might make use of as a weapon? Effectively impossible; you cover the world with padding, then the nutter is just going to use the padding to asphyxiate their victims. The point you need to address is the one in between their ears, not what’s available to them in the surrounding environment.

    • Not every mass murderer is mentally sick. But morally insane people are more common and more likely to be mass killers. Just look at what happened in Buffalo when a racial supremacist shot up a supermarket. He was perfectly sound in mental health but apparently believed that black people were out to use abortion clinics to kill off all white people. I could be wrong.

      • I would submit that once you’ve decided to kill indiscriminately without cause, you’ve pretty much left the bounds of reality and rationality far, far behind.

        Killing someone as an answer to an argument is fundamentally insane, on the face of it. It’s also insane as an answer to questions of “I want their stuff…”, which is what most arguments of this nature boil down to.

        I’m not going to comment on the rationality of any of it, because I think it’s an essentially insane answer to any problem short of that person trying to kill you and then killing them in self-defense. If your answer is killing to anything other than self-defense, then your question was wrong, and framed by an irrational mind.

      • I believe with vast majority mass murderers (so endemic to US, but they pop out in other places too) its always this insanity or “moral insanity” a prevalent drive, and stuff like racial/ethnical goals could be only an excuse.

        Best example is a, few years ago, case of (probably by his account) anti-semite in Germany who “attacked” his local synagogue using various home made weapons and bombs (some of which jammed and failed, fortunately), but, again, fortunately he was stopped in entering and causing mayhem just by massive doors on the building.
        In his frustration, he started dischargeing and shooting random people across the street, results of which 2 were murdered and 2 injured.
        The catch is, of course, both of these victims were non-jews, nor did they had any connections to such.
        So it shows that in the mind of this insane person, there was a classic spree killer hatred for all of humanity, and proclaimed anti-semitism was only a thin veil or a convenient wrapper, but not ultimate madmans id-repressed goal.

    • Kirk, your comments on “constructive possession” is right on for broken firearms that full-auto and unregistered DIAS! The Constructive Possession Rule also applies to other firearm areas like possessing a too short AR barrel and a AR-15 would amount to having a unregistered SBR (short barreled rifle)”. The more good owners read the law and follow the law, the better we will all be. All firearm owners should be as diligent as you! TY!

      • This is backwards. Martin Luther King, Jr didn’t overturn Segregation by following the law, he led resistance to unjust laws. The NFA is an unjust taking of your Constitutional Rights, and the more good gun owners resist unConstitutional laws, the better we will all be.

  5. Reminder traitor to the American republics Wayne LaPierre conned the NRA’s membership to get the Hughes passed. He claimed in The Monitor there’d be immediate legislation to repeal it to stop opposition to the FoPA, then after decades of no-shows on this promised legislation, LaPierre admitted in 2016 on Face the Nation he always supported it.

    • LaPierre could not have hurt the NRA more by the actions that he has long taken. He lives like a king, NRA has spent 100’s of thousands just for his wardrobe. There are something like over 100 people in the board of directors. This means that no one director can accomplish anything they are too weak.
      I’ve given up on the NRA. Many other organizations exist to support the Second Amendment.

  6. Oh wow, I just had to write an essay about the history of the DIAS for my history class, and I learned so much about the evolution of gun regulation! It’s fascinating to see how small changes to the design of a firearm can have such significant legal implications. I was surprised to learn that possession of full-auto fire control parts was not regulated initially, and that the DIAS was actually widely sold by mail order before it was redefined as a machine gun. It’s also interesting to note that registered DIASs can be worth so much money due to their legal status. It was a very interesting topic but a little bit hard for me, thanks to this site where I found most of the information about it. This topic certainly highlights the complexities and controversies surrounding gun control and regulation.

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