Ask Ian: Why Don’t More Rifles Have Captive Pins?

From Ryan on Patreon:
“Why do more rifles not have captive takedown pins? It seems that if someone is developing a rifle from the ground up there are almost no drawbacks to having them, especially if the rifle is going to be widely fielded. Is there a hidden cost?”

Having captive pins does add a few more small parts, machining operations, and assembly complexity to a rifle design. However, more important is that not all materials are conducive to captive pins. There needs to be a way to house the spring and detent pin necessary to create a captive takedown pin, and materials like stamped sheet metal and polymer are not usually well suited to that. Stamped steel has no space, and drilling the necessary holes in this polymer walls creates weak points.


  1. Retention built into the pin makes the most sense for front pins to me; the FAL / MAC type seems fairly hard to jam. I don’t deny that it’s a possibility, but all you lose on a two-pin setup is the ability to swap uppers (a relatively rare occurrence, especially in the military). Even then it’s temporary, and can be fixed at the armory (where you’d swap uppers anyway).

    I needed to relocate the front pivot point on a rifle I’m customizing, so I’m using a shoulder screw and Loctite because I don’t foresee swapping uppers (especially since it uses interchangeable barrels).

  2. Couple of points… One, if there’s any taper at all on the rear takedown pin spring/detent tunnel on the AR-15 or M16 series of rifles, I’ve yet to discern one. I’ve done the “take the rifle completely apart” drill several thousand times as an armorer in the Army, and I do not ever recall encountering that described feature. Indeed, I’ve often had the detent exit the front of said tunnel once I’ve released the tension on said spring, and the rear pin has fallen out.

    Also, can’t see any sign of a taper at all in my mechanical drawings of the lower receiver that I’ve got, so I’m more than a little puzzled at Ian’s description. If that taper is there in the spec and the delivered weapons, it’s obviously so slight as to be useless as far as detent retention, because every single M16 I’ve ever worked on would have had to be so worn as to make it non-functional.

    That quibble aside, the choice between “captive takedown pin” and “free takedown pin” isn’t always due to materials or what have you; it’s often due to what might be best referred to as “cultural” reasons. The G3 is what it is partially because that’s what they used on the StG44, and the later StG45 prototypes. Continuity of design/interface, donchaknow?

    If a designer or a military has faith in the idea of a “free pin”, they’ll use it. If not, well, you’ll find them doing things like FN did with the FAL, and that front pin ain’t coming out unless you’re the armorer, and the rear takedown pin is actually gonna be a latch. Some military/industrial cultures just don’t trust the troops not to lose pins or other small parts. Perhaps, rightly so.

    I’d submit to you that if a designer wants captive pins, they’re going to design them in, regardless of material. Or, they’ll eschew pins entirely, as Kalishnikov mostly did with his designs. If a military thinks captive pins are mandatory, that will be part of their specifications for the weapons they buy; if they’re indifferent to the issue, they won’t care, one way or another. I suspect it has a lot to do with people like me, who spent years dealing with the troops; you come from a background where lost pins were one of your bete noir frustrations, well… You get a chance to write procurement specs, guess what you’re gonna do?

    It’s a cultural thing, in a lot of ways. Some are OK with non-captive, some aren’t. I suspect it has a lot to do with the sort of people you’re taking in as soldiers, because if they’re mechanically inept and prone to losing small parts? The pins, if there are any, are gonna be captive. Or, you’re not going to have functional weapons, after very long…

    An Israeli once told me that the pin issue was a major factor in why they went with the FN, rather than the G3. They supposedly could have gotten the G3 in a very nice deal from the Germans, but they decided not to after looking the design over and deciding that the Israeli conscripts would probably drive everyone mad losing pins… Now, I’ll have to tell you that this is purest apocrypha, because I’ve never ever even seen the slightest indication that the G3 was looked at by the IDF in any serious way, so take that for what it is worth… One Israeli telling me something with zero supporting documentation. Would not surprise me either way, but I’ve never seen or heard of that “G3 for Israel” thing anywhere else.

    • Actually I have never really heard of lost takerdown pins on for the G3 or G36. It will have happened, because it is possible. But I have never really heard of it. Making me think it is not a big problem. Having an easy built in stowage places for the removed pins to put may have helped. Being used to the removeable pins, I find the captive pins a bit annoying sticking out, while handling the disassembled pieces of a rifle. Another advantae of the “HK” style pins ist, that you can push them out with your fingers and needs no tools, whereas most captive pins I have seen have to be pushed with some type of tool. And the often referred to bullet tip tool makes me uneasy, because it is life ammo near a rifle you do not want to shoot in this moment.

      I think this comes down to taste and what one is accustomed to in the end.

      • You’re German. Of course you and your fellow Bundeswehr conscripts wouldn’t lose the pins… The sort of Germanic mentality that would chose such a thing in a weapon would also be the sort of mentality that wouldn’t lose them in the first place.

        However, I understand that that issue isn’t exactly a non-issue in other regions of the world. I remember hearing a South African bitching about the SADF giving G3 rifles to their clients in Namibia and Angola, instead of the R1, which was reserved for South African formations. He said the Angolans were horrible about losing the pins (and, other things, like cleaning kits…), and that it was a constant source of worry for all the trainers and advisers they had.

        Granted, one guy, anecdotal, but… There are a lot of assumptions baked into designs meant for a certain audience, which may or may not also work for others. In my case, I found that a lot of the fussy little pins on the M60 were common enough loss items that I kept a stock of the bastards in reserve wherever I went, especially when they brought in the Small Arms Repair Parts restrictions back in the 1990s. Same-same with the M16, whose design I would frankly have to give an “F” to, for that aspect. The worst offender, I found, was that damn extractor pin that guys would keep losing. I remember one exercise where I went through about six of those bastards in my platoon, which was highly annoying; I was having to source them down at the local gun show, and the price was insane, like three dollars a pop.

    • Great write up, but Jesus Christ man, you cannot possibly in this day and age write such gun blasphemy like wrong letter in Kalashnikov (sur)name ;-))

  3. One solution is to forgo pins altogether in favor of a spring tab, where let’s say the back end slides a quarter-inch into the receiver where it is prevented from moving either laterally or vertically, while at the same time the front end slides into the receiver where it is also restrained laterally and vertically, but a ledge also rides over a spring tab and snaps into lock. The lift portion of the tab would be protected by recessing it into the front of the receiver to prevent accidental disassembly. Everything can be made from stamped steel, and the spring tab can be one riveted piece of spring steel.

    • On a similar note, you could use spring tension in the back (as the XCR uses the tail of the recoil spring to control pivoting) and a fixed lower pin coupled with an open slot in the upper (like a single or double shotgun) in the front.

    • I’m a huge fan of tool-less takedown, and if you use pins, then you’re almost always going to find yourself in a position where fouling, dirt, or who-knows-what has made those pins nearly impossible to remove absent a punch and a hammer.

      I am also a fan of the idea that tools necessary ought to be as minimal as possible, and that if an echelon of maintenance is not supposed to do that work, then they should not have those tools issued to them. I’d go so far as to put restrictions on things such that operators have standard hex nuts of set minimal number on the things they’re supposed to work on, that the next higher level of organizational maintenance has a totally different style of fastener like Torx or whathaveyou, and that the things left to depot-level work are fastened with a set of fasteners that neither operator or organizational maintenance get issued to them. If nothing else, the fact that someone has to go get a different set of tools to work on some assembly would serve as a signal that “Hey, maybe I shouldn’t be screwing around with this…?”

      Frankly, in a lot of ways? The use of pins in small arms, especially ones that are not captive, is a sign your design is not mature enough to field. If you’ve got them in your design, you need to go back to your drawing board and reconsider a lot of things.

      I am not really a fan, TBH. Captive pins are entirely too prone to issues with dirt, non-captive pins are equally susceptible, and they have the added fact that they’re all too easy to lose, rendering the weapon non-functional. Color me in as a big believer in things like the Valmet interpretation of the AK-series, wherein it’s all pin-less for takedown.

      Also, color me in as a non-fan of the fussy little pins and cam pins in the bolt assemblies of the AR-15 series of rifles and the entire oeuvre of the AR-18 pattern. If you’re expecting little Private Johnny Fumblefingers not to screw things up by losing leetle fussy parts, then don’t design them in. Eugene Stoner, I’m looking at you and your freakin’ extractor pin, cam pin, and firing pin retention system. Kalishnikov did the bolt system right

      • Mikhail Timofeyvich did pretty much everything right. Compared to the designers of the MKb42-StG44 “family”, who did most everything “Teutonically”.

        As for Eugene Stoner, he did everything based on the principles of aeronautical engineering. Because that was what his employers, Fairchild-Republic Aircraft, Inc., were used to manufacturing.

        Which is sort of like having your pop-up toaster built by a shipbuilder. Imagine a Toastmaster built by Blohm & Voss.

        Of course, the Luftwaffe knew what it was like to have them building aircraft. See BV 141.



        • You’re gonna laugh, but I understand that Blohm & Voss did have a line of household appliances at some point after one of the wars… I don’t know if they built toasters, but I wouldn’t be surprised.

          I don’t know that Stoner necessarily built the AR-10 the way he did because “aviation engineering/manufacture”. If you look at the other things he designed, before he went to work for Fairchild, there were signs of things to come in his future, including all that forged aluminum. Then, when you look at the AR-16, which became the AR-18, it’s pretty clear that he wasn’t necessarily wedded to the idea of copying aircraft engineering…

          It’s all a bit of muddle; I still haven’t seen where/who/how they came about the ergonomic design for the AR-10, and believe me, I’ve looked. I’d love to know exactly who was responsible for that, and how they reached their design formula.

          If anyone had a chance to ask Stoner or Sullivan, I’ve yet to see it fully explicated…

  4. AUG takedown pin is also a sling swivel, so in practical use grunts can’t loose it because the sling is attached at the other end of the gun

  5. Ian is correct – making captive pins is costly, and lets face it – vulnerable to damage when disengaged.

    I had that opportunity to study in detail (as part of my occupation) captive pins at several rifles; I was never really enthusiastic about them. On AR15 especially it is a finicky undertaking. I consider a smartly designed latch type closure a far better solution. In most cases it removes undesired motion between subassemblies.

    • There is another reason AGAINST captive (or loosely inserted) pins consideration and his is inescapable. If you have two pins connecting two halves of rifle, such as on AR15 you need to provide tolerances for holes location and sizes variation plus thickness and variation of coating. This ends up invariably with slap (loose movement) between them. Resulting feel is shoddy.

      I recollect my first contact with AR5. I could not help it but ended up with comment: “what a PoS is that?!” Well, they fixed it afterwards with a flexible insert. But, I’d still prefer that feel of firmness between what I hold and at what the sights are mounted on. It should be one and the same piece.

      • I have to agree with you. I still think they should have gone with a chassis-style design, somewhat like what Madsen did with their attempt at an assault rifle. I think that the barrel and operating system/bolt ought to be firmly and permanently affixed to the “holdy-on bits” that interface with the shooter, and that the idea of “shotgunning” the weapon a la the FN FAL or AR-10 is anathema.

        My first M16 in Basic Training was a GM Hydramatic product. There was easily a half-inch of play when you went to apply any pressure at all to the upper receiver/lower receiver interface. I still shot expert with it during pre-qualification, and would have likely shot expert with it during actual qualification, but the rain that day, when coupled with glasses and the peep sights…? I was lucky to make sharpshooter.

        On the M16, that slop is really not much more than an aesthetic issue; it feels bad, you don’t like it, but when your sights are on the upper and so long as the slop doesn’t result in misalignment of the bolt carrier with the buffer tube, it’s an entirely immaterial issue so far as the accuracy and reliability of the weapon goes. Hell, it might even be a positive thing, because all that slop allows a bunch of fouling and dirt to get blown out of the receiver during shooting…

  6. I agree with Kirk. The fewer specialized tools required for assembling or disassembling a gun, the less likely someone will foul up the job. Unfortunately, we can’t always cure intentional stupidity and thus we shall have to allow some fools to learn the hard way, preferably with a mashed finger instead of a missing limb or eyeball.

    • “Thou shalt not remove the buttstock retaining pins unless the bolt be all the way forward.”

      One of the Ten Commandments of M60 GPMG takedown.



      • As a young and very inexperienced armorer, I may have taken the backplate off of a cocked .50 caliber M2HB. Not one of my smartest moves, but the manuals I had ordered hadn’t shown up, and there was an inspection coming, so I figured, “Hey, what could go wrong…?” when it came to teaching myself the weapon.

        Since God looks after idiots, when the recoil spring and guide took off, they only brushed my ribcage lightly, putting two neat holes into my BDU blouse and T-shirt. The fact that said assembly completed its flight plan by penetrating the cinderblock wall behind my workbench served as food for thought, let me tell you. It also demonstrated that whoever had built the arms room for the Army had neglected to fill in the voids in said cinderblock wall, which was a point of interest to the people in charge of assuring the security of said arms room…

        Interesting day with some interesting phone calls, that was.

        • The NG Master Sergeant (Marine MGSgt otherwise) who taught me the M2HB strongly advised staying to one side or the other when removing that backplate, even with the bolt forward. As he put it, standing directly behind the Deuce while dismantling was only marginally less dangerous than being directly in front of it any other time.



    • “(…)missing limb(…)”
      Once upon a time there was high accident rate when servicing KPVT.
      Designer,_%D0%A1%D0%B5%D0%BC%D1%91%D0%BD_%D0%92%D0%BB%D0%B0%D0%B4%D0%B8%D0%BC%D0%B8%D1%80%D0%BE%D0%B2%D0%B8%D1%87 was instructed to research that. He managed it so effectively that he killed himself in process of doing that after releasing spring.
      Nowadays KPVT manual contain following passage
      Разборку, сборку, чистку и смазывание пулемета следует производить под наблюдением командира боевой мащинь
      That is
      Disassembly, assembly, cleaning and greasing of machine gun is done under supervision of commander of vehicle

    • I think there are some corollaries that ought to be taken into account with design, as well–Namely, that weapon takedown should not be actively dangerous to the person taking the weapon apart. M2HB, M60, Glock… All of these demonstrate safety issues in field-stripping, issues which should have been designed out before ever seeing the light of day.

      Basically, if you’re writing a manual and find yourself saying something like “It is essential that you do X before Y, or you might put an operating rod through your chest wall…”, you probably need to go back and rethink a lot of your entire design. I’m all up for booby-trapping things for the enemy, and I do think we need to weed out a lot of the dimmer members of the gene pool, but I’m of a mind that a good weapon design does not include esoteric little hazards that will likely entrap the neophyte or the untrained… In other words, if you have two pages of safety precautions referencing takedown procedures before the body of your operator’s manual, you might just be doing it wrong.

  7. My experience with no captive pins is limited. A half hour looking for a G3 pin when the German instructor dropped one on the range. (Finely mowed grass). Being told to take a small bag of finishing nails to Vietnam as solders where lousing cotter pins faster than the Army could replace them

Leave a Reply

Your email address will not be published.