Closed vs. Open Bolt technology

Screenshot 2014-01-06 00.56.15It’s generally understood that weapons that fire from an open bolt are less accurate than those that fire from a closed bolt. It’s certainly logical that both the weight of the moving bolt, plus the inevitably longer locktime, will increase dispersion. An interesting controlled experiment the US Army conducted in 1978 confirmed the existence of, and tried to quantify, this increase in dispersion. Additional findings were that:

  • Not just the dispersion, but also the precision of the shots suffered. That is to say, not only their separation one from the next in the same group, but their separation from the intended point of aim.
  • Comparing prone unsupported to the less accurate and consistent offhand position, the penalty for using an open bolt was greater in the offhand (this might be predicted logically).
  • Right-handed shooters were consistently off target to the right, and this increased when firing from an open bolt. Lefties erred in the opposite direction.

Unfortunately the only usable data come from the dry-firing tests, in which a laser and receptor were used. This methodology could not be validated by live-fire testing for an interesting reason: the innate accuracy of the weapon they used, the XM19 SPIW, was so poor.

For the purpose of the test they had a single XM19 and a quantity of XM645 flechette ammunition. The test weapon was Serial Number 6 and was lent to the Ballistics Research Laboratory by the manufacturer, Aircraft Armaments Incorporated of Maryland.

THE XM19 was AAI’s entrant in the Special Purpose Individual Weapon contest in the 1960s, and the follow-on Future Rifle Program of the 1970s. It fired flechettes at a very high cyclic rate and was intended to mate with a grenade launcher (the M203 was an outgrowth of the first stage AAI’s GL development; a second-stage repeating launcher was not a success). It had extreme reliability problems; while then-current service weapons, like the M14 and M16, could fire thousands of rounds without a stoppage, the XM19 never managed more than dozens. (In a profile of AAI founder/designer Win Barr for Small Arms Review, George Kontis wrote that bad single-shot accuracy at range was what killed the AAI flechette rifle).

The XM19 was also badly designed ergonomically, awkward, overweight, and muzzle-heavy even without the grenade launcher (which was not installed during the 1978 tests). Why this test was not done with a more conventional weapon is unclear. It may have simply been opportunistic, in that both open- and closed-bolt lockwork for the XM19 was readily at hand.

These facts suggest a follow-on experiment. It would be worthwhile to repeat the test but with a more modern alternative. For example, if you got 20 marksmen to fire 10-shot groups each with an AR clone rigged to fire from open and closed bolt.

That’s a problem with the ATF, which defines any open-bolt semiauto as a machine gun, no matter how robust the disconnector is. But the work-around is to use a lower with no magazine well at all. A single-shot weapon can’t be a machine gun, no matter how intently the ATF tries to find a technical violation to fry somebody for.

The original, primary-source document. Technical Memorandum 2-79, Aiming Point Displacement from Firing a Rifle from the Open-Bolt Position by Dominick J. Giordano, is available:

  1. From DTIC:  Abstract.  Full document (.pdf)  (Our apologies for the poor resolution of the photos, DTIC appears to pull these from microfiche, which yields an equivalent depth of 1-bit per pixel).
  2. From right here at  ADA068215 .pdf

7 thoughts on “Closed vs. Open Bolt technology

  1. Law of Self Defense

    Love the blog including this post, but I think you misuse “precision” in your first bullet point here.

    What you seem to be describing–variability with reference to the point of aim–refers to “accuracy”. Even a wide shot distribution, if centered on the point of aim, would correctly be termed “accurate”. In contrast, “precision” refers to the variability of the shot group without reference to the point of aim. A tight shot distribution, even if badly offset from the point of aim, would correctly be termed “precise”. Ideally, of course, we would like our weapons to be both accurate and precise, consistent with practical requirements and limitations.

    Or perhaps I just misread what was written. :-) In any case, keep up the great work.

    –Andrew, @LawSelfDefense

  2. Stefan van der Borght

    Great, you prodded the engineering cells awake again. Thanks. How about a counterweight that cancels out the accuracy-killing jolt of the bolt, kind of like the scrumptious AN94? Ok, more weight and complexity, but we don’t have to carry it or pay for it, and we don’t have to go as radical as the two-shot wunderkind from Izhmash:

    Now I’ll get back to my micro-crossbow challenge, whichI’ve just about solved up to prototyping stage. Methinks a telepathic-datalink 3D printer would be a nice birthday prezzie for me….wonder if Auntie is feeling generous this year?

    1. Hognose Post author

      Since you’re in Der Vaterland, can you link up with Jörg Sprave? Jörg has Kid’s favorite YouTube channel.

      The open bolt has some important benefits, principally for cooling. With the M60 (kind of like a Porsche 911 in that its functioning is a triumph of dogged engineering over bad initial design decisions) they were able to keep barrels alive and prevent cook-offs a lot longer than aircooled brownings by firing from an open bolt and having a clear air gap between the chamber area (which is not the hottest area thanks to its cyclical opening to fresh air) and the tube. The Stellite liner also serves durability, if not cooling (the chamber is not lined, the liner begins at the leade).

      Some designs fire from open bolt in full-auto for cooling, and closed bolt in semi for accuracy. FG42 did this. M60 copied FG42 which copied Lewis Gun (Lewis was not selective fire, full-auto only).

  3. Stefan van der Borght

    Until someone invents economic barrel and projectile materials that can handle the heat and keep their shape & strength we’re stuck with open-bolts, barrel swapping, and crappy first shots, or we solve the cooling, or come up with some funky mechanical workaround. I wonder if the cookoff-resistant propellant they made for the G11 spun off into anything useful? A counterweight lighter than the bolt but moving faster and in the opposite direction should cancel out the reaction; or, a heavier one moving slower (barrel itself?) That way one could have accurate single shots which is one of the two disadvantages of open bolters mentioned in the study. There must be a reason nobody bothers with this, but I can’t nail down a definitive one from the persistent spectres floating about me (go away sucky HK, I hate you too). The AN94 has a nice double tap feature but it needs the energy off the first shot and so has to be closed bolt; and it’s too complicated and expensive (and I still want one). It at least shows there are solutions to be found. How come they didn’t use something like the FG42 for testing, since it has a much better axis alignment to the shooter and wouldn’t suffer as much from pitch?

    Since Mr Spräde lives only 40k away I’ll see about an outing sometime soon. He seems an approachable type and we also seem to have similar interests and outlooks. I’ll expedite the protoyping of the micro x-bow and get the first production run done asap, and then see if the SlingshotMan will autograph one for your Kid (is that a suitable addressee nickname for the ‘graph, or would something else be more suitable?) Must also think of a suitably anonymous way of sending the item if this comes off, this being the internet and all.

    Design parameters are: a single prod of loose-laminate tempered spring steel, colour case hardened tiller, rust blued folding grip with old growth Tasmanian Oak inserts, rust blued bolt shroud/foresight, simplified nut & single stage trigger with grip safety, bolt dimensions 5mm x 70mm aluminium with steel vanes & tips, max dimensions 25mm x 97mm (it all has to fit in a flat round tin for safe portability). Plinking, From office desk, For the purpose of; and gifts for folks I take a liking to. Since Mr Spräde hinted at developing a taste for steampunk I’ll slap some brass on it as well, nothing kitschy though. Your youngster likely has a birthday coming up, but why wait. (Please delete these last 2 paragraphs so Son of WM doesn’t get his surprise spoilt).

  4. Kirk

    I gotta be brutally honest. I think that until the materials technology matches the requirements, caseless is never, ever going to work. It’s been the technology of tomorrow just as long as the flechette, and neither one of them has a chance in hell of materializing as an effective weapon in my lifetime. Likely not in the lifetime of any kids I might have, either.

    The money spent pursuing these wil-o-the-wisp ideas would be better spent in providing better training and more ammunition for live fire than it will ever produce being spent on these fantasies of the developers.

    I would love to have the design effort and money we’ve wasted on all this stupid shit back, and use it to pay for more training, better evolutionary design processes, and more testing of existing weapons.

    The G11 is a typical failed system. I ran into a German officer who’d had something to do with the testing of that thing for the Bundeswehr, and it was his contention that the only reason they were talking about it being adopted boiled down to the choke-hold HK had on Bonn. The Bundeswehr didn’t want the damn thing unless it worked, and it never, ever worked. The ammo was too fragile, too sensitive to storage conditions, and the list of issues they had with it never got really got fixed. Talked to the guy, filed the conversation away, and then we heard the same thing from the guys who participated in the ACR test at Benning back in the early 1990s. That caseless technology that HK claims was “perfected” back in the 1980s was sold to ATK, and I’m pretty sure they feel like they were ripped off, because it still won’t work for them with the new LSAT program’s caseless track.

    There’s a paradigm-changing technology out there. Trouble is, nobody knows what it is, or how it’s going to work. Until it shows up, and is identified, why don’t we concentrate on doing things that we know will improve our lethality? Like better training, and more ammo to do it with?

  5. Stefan van der Borght

    Agreed, the G11 was a white elephant, good for someone’s business but no good for what it was supposed to do. I was just curious as to whether at least Dynamit Nobel spun off any benefits from the propellant they developed for it. Why aren’t militaries more sensible about their weapons? For example the Pommies could have stuck with the EM2, the BREN (switched to belt form) and the .280 back in the 50’s and avoided the FAL & SA80 altogether. They already had the BESA in 7.92 so they could have kept that excellent round for their TMG & MMG. But then, it’s about money, not winning; at least not for the peons, or even the pawns. I cried when “Flakjacket” Johnny Howard had all our lovely Lithgow-built SLRs melted down. No warstocks, no danger of the Aussies ever wising up and kicking the sellout scumbags in Canberra out. Billy Hughes and the Anzacs would be furious if they knew how their achievement was stolen. Then again so would today’s Aussies if they got off the footy & beer & McMansions, but that’s neither here nor there and nothing to do with open bolts. Metalstorm is interesting because it avoids all the clunky moving bits altogether, but it seems to be stalled. Perhaps the Chinese will run with it. Fanatastic, all we’ve got left is the AUG, the F18 and all that open space. Even the Boers must be laughing, almost as loud as Mr Tally Ban and his Khyber Pass .303 at the M4, good as it is at what it is meant for. The new paradigm is robots, giant and tiny. They don’t tire and never question their orders no matter how illegal. We meaties are going to have a hard match.

    1. Hognose Post author

      Believe me, the Taliban are not using we and fields Lee-Enfields (curse you dictation software), and haven’t for decades. Their long-range weapon, such as it is, is the PKM.

      The coalition owns long-range in the hills and mountains of Afghanistan. It isn’t westerners who get whacked at 2500 m.

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