Ask Ian: Why Does the AR15 Have a Buffer Thingie?

From Arvid on Utreon:
“Why does an AR-15 need a buffer thingy? Why can’t it just have a spring like every other normal gun?”

The Ar-15 really doesn’t need the buffer and tube, but it is a holdover form the origins of the system: the AR-10. The intent of the AR-10 was to create a 7.62x51mm battle rifle that was very lightweight (under 7 pounds, originally) but still soft-shooting and controllable. In order to do that, Eguene Stoner. has to pull out all sorts of tricks. As it applied to our question today, this included a straight-line design with a buffer on the end of the bolt carrier to absorb any residual impact of the bolt carrier on the end of the receiver tube. At this time, there was no apparent need to allow for a folding stock, so the bolt was allowed to run the full length of the stock to minimize felt recoil.

After the basic design was put in place, the disassembly was changed from sliding together to pivoting, and this required splitting the single very long bolt carrier into what we now recognize as the bolt carrier and the buffer. When the design was scaled down to the AR-15, the basic architecture stayed the same, even though the recoil-reducing elements were not really necessary in the new smaller cartridge.


  1. It is better it does, compare it to other gun. Colt Commando, painted green; honestly, was toy gun like.

    Unlike any other rifle, you could probably have spun it in your hand like .45 Peacemaker… Trick on telly.

    Take the mag out like, open the bolt; give it a hard slap, get someone else to check it is empty… Before 360 degrees like… Probably sensible.

    Unless you don’t like who is possibly behind you. Even then, probably better to pratice doing it like…

  2. We can turn an Ar into a Bullpup, just use the twin tube Johnson lark; But you pull it out I.e. The stock for todays average, Western… Larger targets, for them to gain a cheek weld.

    Long barrels make sense. With steel arsed cases… Optimise the mutha fucker. So bullpub. Ar gas system, light use Ar this that and the other. Not difficult.

  3. Ian ring Karl, you don’t have to agree… I don’t know; Queen Elizabeth was a Lizard really… Pair of you are like Prince William and Harry; try bow and arrows together or something break the ice, Karl might try a 200lb longbow and miss… Likely.

    Karl what are you like with your outragerous opionions. You put on telly.

    Ian, she might be a Lizard; just cuz you went to Leeds armoury you don’t know.

    Drink beer pair of you and make up.

  4. I was told the buffer was used to reduce the cyclic rate of fire from aprx 900 rounds per minute to 600. The idea was preserve ammo depletion.

  5. Ian tries and does rather well. He properly mentions the historical sequences which is important.

    If I could simplify, I’d say following: the upper receiver cavity (tube) measures exactly 1 inch. That by far does not provide for sufficient Bolt carrier mass to recoil at reasonably slow velocity. This is why the mass is added in form of encapsulated buffer.

    The buffer has on top of it weights and soft washers which reduce dynamic response further and thus reduce rate of fire. I have seen entire mathematical analysis on the subject; not by Mr. Stoner but by U.S. Army research office, way after the rifle was created.

    It confirmed why the system works. Stoner worked solely by instinct which proved to be sound. The art of being a designer is to know how to fix the shortcomings.

    • DP, any chance you could find a link please?

      I appreciate Ian’s answer, but he’s managed to do it without really touching on how the AR buffer does its job.

      What proportion of energy it spreads over a longer period of time and what proportion it dissipates as heat and how.

    • The interesting thing is that while Stoner gets a lot of credit for the AR-15/M-16, he really didn’t have all that much to do with the conversion of the AR-10 from 7.62 NATO to 5.56X45. That was mostly Sullivan, Fremont, and a couple of other guys, who were acting at the behest of Stoner and executives within Fairchild. From what I’ve read, Stoner was never enthusiastic about the entire concept of SCHV as expressed by the 5.56X45. He was still a big believer in the full-power large caliber mentality that created the 7.62 NATO in the first place–He was mostly focused on trying to make it work, more than anything else.

      And, he did come a lot closer to it than anyone else. At least, in my opinion.

      I still want to know who the hell it was that did the ergonomics work on the AR-10, because from the documentation I’ve seen, it goes from “thing that looks a conventionally stocked weapon” to full-blown AR-10 with zero actual development or iterative design in between, along the way. Stoner’s pre-Armalite work goes “Conventional M-5, M-6, M-7…” to “Entirely unconventional M-8” that has all the important ergonomic features of the AR-10/AR-15 families.

      I don’t see that as being realistic, so I suspect there were at least some mock-ups showing different approaches for the controls and so forth–But, I’ve never, ever seen any documented. Anywhere. It’s like the AR-10 just appears like Athena from the head of Zeus, ready to go. And, I don’t see that as either realistic or likely. Where the hell is the intermediate work between the initial design concepts with the conventionally-stocked rifles and the end state M-8?

      If it did spring forth from Stoner’s head, fully-formed? That’s ‘effin-ay incredible.

      Which is why I don’t believe in that as a course of events.

      • Look for The Black Rifle/ M16 retrospective by Stevens and Ezell. It is explained there. The designer of M16 mechanism and ergonomics was Robert Fremont. Jim Sullivan mentions his contribution in interview with Ian M. He specifically mentioned Robert’s talent for determining tolerances which is crucial for manufacturing. Jim S. was Robert’s assistant. E.Stoner was primarily involved with ammunition development.

        • I suspect at the time the AR15 was ripening, Stoner was in full swing in his new project being Stoner63. Something like that is not created in two weeks, so he did not even have time to be involved. He wanted to be out of Armalite and on his own.

          • The Stonber 62. The Stoner 63, again, was designed by Sullivan, scaling down what Stoner designed in 7.62.

            In general, yes Stoner had little involvment in the AR15 and none in the AR18. Obviously there were Stoner’s ideas in them, those that had been scaled down, but it’s difficult to determine how many of those ideas were Stoner’s. As Sullivan recalled later, he was a great designer, but not a one-man-show.

  6. Thanks for explaining that theory about AR buffer tube design.
    It sometimes gets silly when other designers copy AR butt stock fashion without understanding the theory behind the fashion.
    The ultimate silliness is the AK-15 with a telescoping butt stock – around a tubular frame – that also side-folds.

    • When you consider that those designers are trying to take advantage of the existing AR-15 ecosystem, in terms of parts and development? It ain’t that bad an idea–Where it gets stupid is when they copy but do it such that their buttstocks are incompatible with existing standards.

      People underestimate the value of an industrial ecosystem. Consider the Mauser bolt-action: How many manufacturers of that basic system were there? How many plants could produce them? How many different modifications of those designs were done, until the final iterative expression of the Kar98k?

      The AR-15 has an entire ecosystem of manufacturers doing things with it that make it a superior system to anything else on the market. Hell, even HK had to finally throw in their roller-locked towels and acknowledge that fact, producing the HK416/417. Which are both basically the rifles designed by Stoner, only with AR-18 gas systems grafted on.

      • Uh, stupid question. Considering that a right-handed magazine release absolutely requires the user to GET HIS FINGER OFF THE TRIGGER in order to start the reloading cycle, how many idiots in the “video game weeb commando” category have demanded that the AR-15 incorporate a mag release that allows the user to keep his finger on the trigger during the reloading cycle so he can get back to shooting things faster? I do speak of the nitwits who apparently think it’s acceptable to bring an assault rifle into a job that requires a belt-fed machine gun.

        • Mag release inside the trigger guard, maybe not, but bolt catch inside the trigger guard? See the euphemistically-named Battery Assist Device (or B.A.D. Lever), which is a sort of a dog-leg thing that reaches down into the front of the trigger guard and allows the right-handed shooter to send the bolt forward by pressing out (ie. to the left) with the tip of the trigger finger. I think it’s aptly named— not just a solution looking for a problem, but a fundamentally unsound one. The only control that belongs inside the trigger guard is the trigger, full stop. There is not and should never be a reason to have a finger inside the trigger guard except when sighted in and deliberately pulling the trigger; any gizmo that contravenes this principle is on the wrong side of the third rule.

  7. According to a couple of books I’ve read recently, the buffer is vitally necessary if the AR has a barrel shorter than 20 inches (about 50 cm). Because that means a shorter gas tube, which means higher gas velocity entering the internal piston in the bolt, which means faster unlocking and higher bolt velocity.

    Faster unlocking means higher cyclic rate on autofire. Higher bolt velocity means greater operating stresses, and broken bolts over time.

    This was first noticed on the XM177 in Vietnam, before anybody ever got worried about the effects of ball powder on the gas system of the M16 itself.

    The answer was the heavier buffer of the M16A1.

    NB; The forward bolt assist (FBA) predates both the heavier buffer and the closed-end flash suppressor. It was introduced in mid-1963, when it was noticed that dimensional quality control on early M193 ball ammunition from contractors, notably Lake City, was erratic. The FBA was not and is not to force a “dirty” cartridge into the chamber; it’s to deal with cartridges whose dimensions are just a bit too close to the upper end of the allowable limits.

    What that has to do with this discussion is that if you couple the wrong kind of propellant (leaving excessive carbonate fouling in the gas tube) with too high bolt velocity (which the heavier buffer was supposed to be a fix for) and cartridges that may be an overly-snug fit in the chamber (which was the reason for the FBA), you get….exactly the kind of stoppages and failures that gave the M16 a bad reputation six decades ago.

    So that buffer needs to be there. Whether Mr. Stoner thought it was needed in a smaller caliber or not.

    IMPO, any rifle firing a small-caliber, high-velocity (over 800 m/s) round with standard pressures of 430MPa (62,500 PSI) plus had better have something to help keep that bolt closed until pressure drops to reasonable opening/extraction levels.

    By itself, the mass of the bolt in any iteration of the Armalite design isn’t going to get the job done. But that’s the price you pay for a lightweight self-loading rifle, no matter what caliber.



    • The buffer is not playing any part in keeping the bolt closed though

      It’s taking the shock out of the end of the bolt travel

      It also has subsidiary roles in shortening that bolt travel,

      And in reducing the bolt velocity for its return. The bolt is having something more like a cushioned stop, rather than bouncing back off a hard surface

      In a way, that does decrease bolt carrier bounce on locking

      • While the hypothesis is that the buffer plays no part in keeping the bolt shut, Newton’s First and Second Laws of Motion say otherwise.

        First Law; Every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This tendency to resist changes in a state of motion is inertia.

        Second Law; The acceleration of an object is dependent upon two variables – the net force acting upon the object and the mass of the object.

        The mass, and thus the inertia, of that buffer resists rearward bolt movement. Even during the unlocking phase of initial recoil.

        If the buffer were behind the recoil spring, it would indeed not be a factor. But being in front of it, it is. Rearward force has to act upon it before it begins to affect the recoil spring. It’s a matter of milliseconds, but it’s still there.



        • Got to admit
          You are correct

          My memory of where the buffer was positioned in the recoil system was completely wrong.

          • We were recently discussing about the fact that there seems to be an ideal ratio (even with ample variations) between the weight of the bolt and that of the bolt carrier. Physically the buffer is part of the bolt carrier, so it helps it to reach the ideal weight without that weight putting stress in the frame when it hits it, at the end of the stroke.
            That was morte important in the AR10, that had to deal with more energy and the same aluminium frame. Without the buffer attached to the carrier, the carrier should have been weighted anyway, and a buffer should have been added to the frame anyway.

Leave a Reply to Pdb Cancel reply

Your email address will not be published.