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Most automatic weapons, with the exception of really weird designs like the Madsen LMG and Hino-Komuro, have a linear reciprocating breech member; either the bolt or a bolt carrier group. This reciprocating member is supposed to move rearward (the recoil stroke) and pull the spent case from the chamber, and then rebound off of a spring to shove a new round into the chamber (the counter-recoil stroke). After the counter-recoil stroke the reciprocating mass should come to a halt in its forward-most position; the "in-battery" position. When the bolt carrier group is in battery the case is entirely surrounded by the walls of the firing chamber and the locking mechanism is fully engaged, so it is safe to fire. Things do not always work ideally, however, and sometimes this reciprocating mass bounces instead of coming to rest. This is called (somewhat erroneously in the case of gas-operated weapons) "bolt-bounce." Andrew Tuohy removed the buffer weights from the buffer in an AR-15 to make the action bouncier for illustrative purposes: There are two ways that bolt carrier rebound can be a problem. In extreme cases the bolt carrier will rebound, but a combination of high friction in the action and weak return springs will mean that the bolt carrier gets stuck and does not go back into battery. Hopefully the designer was smart enough to design the thing so that it absolutely cannot fire when it is out of battery, because out of battery cartridge ignition is an excellent way to convert a firearm into a pipe bomb. If they were so wise, then there will be a failure to fire of some variety. Generally speaking a weapon has to be unusually dirty, worn, or poorly designed for this problem to occur. Return springs are usually strong enough to get the moving parts into battery even if they aren't fully compressed. I have, however, witnessed this problem in German K43 rifles because they are a pile of suck and fail. But they're pretty. The second, more likely problem only rears its ugly head in fully auto fire. In most full auto weapons there is an auto-sear, which a secondary sear which releases the hammer as long as the trigger is depressed. The auto-sear is tripped by the bolt carrier during counter-recoil, usually when or just before the bolt carrier goes into battery. If the bolt carrier rebounds off the front of the receiver and the timing is just wrong, the hammer (or striker) will hit the bolt carrier when it is slightly out of battery. Again, competent designs have means of preventing out of battery ignition and the attendant facial and manual reorganization that tends to go with that. However, when the hammer or striker hits the out of battery bolt carrier its kinetic energy will be spent. This means a failure to fire. Early M16s had rebound problems, particularly during full auto fire. Originally the buffer was intended simply to be a hollow spring guide, but a problem with light primer strikes forced a redesign of this component in 1966. This image, from the patent for the improved buffer shows the series of sliding weights that were added to the buffer. These work like the sliding pellets in a deadblow hammer and arrest the tendency for the bolt carrier to bounce. The additional mass had the added benefit of slowing down the velocity of the bolt carrier, which reduced wear on the parts and lowered the cyclic rate of fire, which improved full auto control. The HK roller-retarded blowback guns, owing to their extremely high bolt carrier velocities, have a strong tendency to rebound unless somehow checked. The solution HK engineers hit on is an anti-rebound claw: Labeled as the "bolt head locking lever" in this diagram. This is a spring-loaded claw mounted on the bolt carrier that grabs the bolt head as the bolt carrier group goes into battery. The lever essentially ratchets into place with friction, providing enough resistance to being re-opened that the bolt carrier does not rebound. The FAMAS, which has a similarly insanely high bolt carrier velocity, solves the problem in a very similar way. In this case, however, the charging handle is the anti-rebound device. The arrangement is similar to the locking catch on an AR-15's charging handle, except that it's much more robust because the catch is responsible for arresting the rebound of the entire bolt carrier. There are other ways still to arrest the rebound of the bolt carrier. The Ruger MP-9 (the one designed by Uziel Gal, not the insanely over complex B&T product of the same name) is supposed to have a spring-cushioning pad at the front of the receiver which brings the bolt to a stop instead of bouncing. The upcoming Desert Tech MDR has, by one account, "an asymmetrical [bolt carrier] face. This is accomplished with a protruding boss on one side of the carrier. As the carrier moves forward to go into battery, the asymmetrical face contacts the barrel extension first. Tolerances within the axial motion of the back end of the carrier group permit the energy to be redirected through a sideways movement. This micro-movement of the rear end of the carrier impedes the bounce and assures full function of the weapon, especially in select-fire operation." Large caliber autocannons often have complex, articulated secondary locks that prevent bolt carrier bounce, since autocannon bolt carriers are enormous and have a great deal of residual kinetic energy. So, when I read that the SIG MCX has some problems with full auto function that sound suspiciously exactly like the same problems the M16 had prior to the addition of the weighted buffer (the same weighted buffer the MCX does away with), I can only roll my eyes. This is nothing new, and there are a half-dozen ways of fixing it. Do your homework.
Here at Sturgeon's House, we do not shy from the wholesale slaughter of sacred cows. That is, of course, provided that they deserve to be slaughtered. The discipline of Military Science has, perhaps unavoidably, created a number of "paper tigers," weapons that are theoretically attractive, but really fail to work in reality. War is a dangerous sort of activity, so most of the discussion of it must, perforce, remain theoretical. Theory and reality will at some point inevitably diverge, and this creates some heartaches for some people. Terminal, in some cases, such as all those American bomber crews who could never complete a tour of duty over Fortress Europe because the pre-war planners had been completely convinced that the defensive armament of the bombers would be sufficient to see them through. In other cases though, the paper tiger is created post-facto, through the repetition of sloppy research without consulting the primary documents. One of the best examples of a paper tiger is the Tiger tank, a design which you would think was nearly invincible in combat from reading the modern hype of it, but in fact could be fairly easily seen off by 75mm armed Shermans, and occasionally killed by scout vehicles. Add to this chronic, never-solved reliability problems, outrageous production costs, and absurd maintenance demands (ten hours to change a single road wheel?), and you have a tank that really just wasn't very good. And so it is time to set the record straight on another historical design whose legend has outgrown its actual merit, the British EM-2: EM-2ology is a sadly under-developed field of study for gun nerds. There is no authoritative book on the history and design of this rifle. Yes, I am aware of the Collector's Grade book on the subject. I've actually read it and it isn't very good. It isn't very long, and it is quite poorly edited, among other sins devoting several pages to reproducing J.B.S. Haldane's essay On Being the Right Size in full. Why?!!?!! On top of that, there's quite a bit of misinformation that gets repeated as gospel. Hopefully, this thread can serve as a collection point for proper scholarship on this interesting, but bad design. Question One: Why do you say that the EM-2 was bad? Is it because you're an American, and you love trashing everything that comes out of Airstrip One? Why won't America love us? We gave you your language! PLEASE LOVE ME! I AM SO LONELY NOW THAT I TOLD THE ENTIRE REST OF EUROPE TO FUCK OFF. Answer: I'm saying the EM-2 was a bad design because it was a bad design. Same as British tanks, really. You lot design decent airplanes, but please leave the tanks, rifles and dentistry to the global superpower across the pond that owns you body and soul. Oh, and leave cars to the Japanese. To be honest, Americans can't do those right either. No, I'm not going to launch into some stupid tirade about how all bullpup assault rifle designs are inherently a poor idea. I would agree with the statement that all such designs have so far been poorly executed, but frankly, very few assault rifles that aren't the AR-15 or AK are worth a damn, so that's hardly surprising. In fact, the length savings that a bullpup design provides are very attractive provided that the designer takes the ergonomic challenges into consideration (and this the EM-2 designers did, with some unique solutions). Actually, there were two problems with the EM-2, and neither had anything to do with being a bullpup. The first problem is that it didn't fucking work, and the second problem is that there was absolutely no way the EM-2 could have been mass-produced without completely re-thinking the design. See this test record for exhaustive documentation of the fact that the EM-2 did not work. Points of note: -In less than ten thousand rounds the headspace of two of the EM-2s increased by .009 and .012 inches. That is an order of magnitude larger than what is usually considered safe tolerances for headspace. -The EM-2 was less reliable than an M1 Garand. Note that, contrary to popular assertion, the EM-2 was not particularly reliable in dust. It was just less unreliable in dust than the other two designs, and that all three were less reliable than an M1 Garand. -The EM-2 was shockingly inaccurate with the ammunition provided and shot 14 MOA at 100 yards. Seriously, look it up, that's what the test says. There are clapped-out AKs buried for years in the Laotian jungle that shoot better than that. -The EM-2 had more parts breakages than any other rifle tested. -The EM-2 had more parts than any other rifle tested. -The fact that the EM-2 had a high bolt carrier velocity and problems with light primer strikes in full auto suggests it was suffering from bolt carrier bounce. As for the gun being completely un-suited to mass production, watch this video: Question Two: But the EM-2 could have been developed into a good weapon system if the meanie-head Yanks hadn't insisted on the 7.62x51mm cartridge, which was too large and powerful for the EM-2 to handle! Anyone who repeats this one is ignorant of how bolt thrust works, and has done zero research on the EM-2. In other words, anyone who says this is stupid and should feel bad for being stupid. The maximum force exerted on the bolt of a firearm is the peak pressure multiplied by the interior area of the cartridge case. You know, like you'd expect given the dimensional identities of force, area and pressure, if you were the sort of person who could do basic dimensional analysis, i.e. not a stupid one. Later version of the British 7mm cartridge had the same case head diameter as the 7.62x51mm NATO, so converting the design to fire the larger ammunition was not only possible but was actually done. In fact, most the EM-2s made were in 7.62x51mm. It was even possible to chamber the EM-2 in .30-06. I'm not going to say that this was because the basic action was strong enough to handle the 7x43mm, and therefore also strong enough to handle the 7.62x51mm NATO, because the headspace problems encountered in the 1950 test show that it really wasn't up to snuff with the weaker ammunition. But I think it's fair to say that the EM-2 was roughly equally as capable of bashing itself to pieces in 7mm, 7.62 NATO or .30-06 flavor. Question Three: You're being mean and intentionally provocative. Didn't you say that there were some good things about the design? I did imply that there were some good aspects of the design, but I was lying. Actually, there's only one good idea in the entire design. But it's a really good idea, and I'm actually surprised that nobody has copied it. If you look at the patent, you can see that the magazine catch is extremely complicated. However, per the US Army test report the magazine and magazine catch design were robust and reliable. What makes the EM-2 special is how the bolt behaves during a reload. Like many rifles, the EM-2 has a tab on the magazine follower that pushes up the bolt catch in the receiver. This locks the bolt open after the last shot, which helps to inform the soldier that the rifle is empty. This part is nothing special; AR-15s, SKSs, FALs and many other rifles do this. What is special is what happens when a fresh magazine is inserted. There is an additional lever in each magazine that is pushed by the magazine follower when the follower is in the top position of the magazine. This lever will trip the bolt catch of the rifle provided that the follower is not in the top position; i.e. if the magazine has any ammunition in it. This means that the reload drill for an EM-2 is to fire the rifle until it is empty and the bolt locks back, then pull out the empty magazine, and put in a fresh one. That's it; no fussing with the charging handle, no hitting a bolt release. When the first magazine runs empty the bolt gets locked open, and as soon as a loaded one is inserted the bolt closes itself again. This is a very good solution to the problem of fast reloads in a bullpup (or any other firearm). It's so clever that I'm actually surprised that nobody has copied it. Question Four: But what about the intermediate cartridge the EM-2 fired? Doesn't that represent a lost opportunity vis a vis the too powerful 7.62 NATO? Sort of, but not really. The 7mm ammunition the EM-2 fired went through several iterations, becoming increasingly powerful. The earliest versions of the 7mm ammunition had similar ballistics to Soviet 7.62x39mm, while the last versions were only a hair less powerful than 7.62x51mm NATO. As for the 7mm ammunition having some optimum balance between weight, recoil and trajectory, I'm skeptical. The bullets the 7mm cartridges used were not particularly aerodynamic, so while they enjoyed good sectional density and (in the earlier stages) moderate recoil, it's not like they were getting everything they could have out of the design. note the flat base In addition, the .280 ammunition was miserably inaccurate. Check the US rifle tests; the .280 chambered proto-FAL couldn't hit anything either.
At the end of January, 2018 and after many false starts, the Russian military formally announced the limited adoption of the AEK-971 and AEK-973 rifles. These rifles feature an unusual counterbalanced breech mechanism which is intended to improve handling, especially during full auto fire. While exotic outside of Russia, these counter-balanced rifles are not at all new. In fact, the 2018 adoption of the AEK-971 represents the first success of a rifle concept that has been around for a some time. Earliest Origins Animated diagram of the AK-107/108 Balanced action recoil systems (BARS) work by accelerating a mass in the opposite direction of the bolt carrier. The countermass is of similar mass to the bolt carrier and synchronized to move in the opposite direction by a rack and pinion. This cancels out some, but not all of the impulses associated with self-loading actions. But more on that later. Long before Soviet small arms engineers began experimenting with BARS, a number of production weapons featured synchronized masses moving in opposite directions. Generally speaking, any stabilization that these actions provided was an incidental benefit. Rather, these designs were either attempts to get around patents, or very early developments in the history of autoloading weapons when the design best practices had not been standardized yet. These designs featured a forward-moving gas trap that, of necessity, needed its motion converted into rearward motion by either a lever or rack and pinion. The French St. Etienne Machine Gun The Danish Bang rifle At around the same time, inventors started toying with the idea of using synchronized counter-masses deliberately to cancel out recoil impulses. The earliest patent for such a design comes from 1908 from obscure firearms designer Ludwig Mertens: More information on these early developments is in this article on the matter by Max Popenker. Soviet designers began investigating the BARS concept in earnest in the early 1970s. This is worth noting; these early BARS rifles were actually trialed against the AK-74. The AL-7 rifle, a BARS rifle from the early 1970s The Soviet military chose the more mechanically orthodox AK-74 as a stopgap measure in order to get a small-caliber, high-velocity rifle to the front lines as quickly as possible. Of course, the thing about stopgap weapons is that they always end up hanging around longer than intended, and forty four years later Russian troops are still equipped with the AK-74. A small number of submachine gun prototypes with a BARS-like system were trialed, but not mass-produced. The gas operated action of a rifle can be balanced with a fairly small synchronizer rack and pinion, but the blowback action of a submachine gun requires a fairly large and massive synchronizer gear or lever. This is because in a gas operated rifle a second gas piston can be attached to the countermass, thereby unloading the synchronizer gear. There are three BARS designs of note from Russia: AK-107/AK-108 The AK-107 and AK-108 are BARS rifles in 5.45x39mm and 5.56x45mm respectively. These rifles are products of the Kalashnikov design bureau and Izmash factory, now Kalashnikov Concern. Internally they are very similar to an AK, only with the countermass and synchronizer unit situated above the bolt carrier group. Close up of synchronizer and dual return spring assemblies This is configuration is almost identical to the AL-7 design of the early 1970s. Like the more conventional AK-100 series, the AK-107/AK-108 were offered for export during the late 1990s and early 2000s, but they failed to attract any customers. The furniture is very similar to the AK-100 series, and indeed the only obvious external difference is the long tube protruding from the gas block and bridging the gap to the front sight. The AK-107 has re-emerged recently as the Saiga 107, a rifle clearly intended for competitive shooting events like 3-gun. AEK-971 The rival Kovrov design bureau was only slightly behind the Kalashnikov design bureau in exploring the BARS concept. Their earliest prototype featuring the system, the SA-006 (also transliterated as CA-006) also dates from the early 1970s. Chief designer Sergey Koksharov refined this design into the AEK-971. The chief refinement of his design over the first-generation balanced action prototypes from the early 1970s is that the countermass sits inside the bolt carrier, rather than being stacked on top of it. This is a more compact installation of the mechanism, but otherwise accomplishes the same thing. Moving parts group of the AEK-971 The early AEK-971 had a triangular metal buttstock and a Kalashnikov-style safety lever on the right side of the rifle. In this guise the rifle competed unsuccessfully with Nikonov's AN-94 design in the Abakan competition. Considering that a relative handful of AN-94s were ever produced, this was perhaps not a terrible loss for the Kovrov design bureau. After the end of the Soviet Union, the AEK-971 design was picked up by the Degtyarev factory, itself a division of the state-owned Rostec. The Degtyarev factory would unsuccessfully try to make sales of the weapon for the next twenty four years. In the meantime, they made some small refinements to the rifle. The Kalashnikov-style safety lever was deleted and replaced with a thumb safety on the left side of the receiver. Later on the Degtyarev factory caught HK fever, and a very HK-esque sliding metal stock was added in addition to a very HK-esque rear sight. The thumb safety lever was also made ambidextrous. The handguard was changed a few times. Still, reception to the rifle was lukewarm. The 2018 announcement that the rifle would be procured in limited numbers alongside more conventional AK rifles is not exactly a coup. The numbers bought are likely to be very low. A 5.56mm AEK-972 and 7.62x39mm AEK-973 also exist. The newest version of the rifle has been referred to as A-545. AKB and AKB-1 AKB-1 AKB AKB, closeup of the receiver The AKB and AKB-1 are a pair of painfully obscure designs designed by Viktor Kalashnikov, Mikhail Kalashnikov's son. The later AKB-1 is the more conservative of the two, while the AKB is quite wild. Both rifles use a more or less conventional AK type bolt carrier, but the AKB uses the barrel as the countermass. That's right; the entire barrel shoots forward while the bolt carrier moves back! This unusual arrangement also allowed for an extremely high cyclic rate of fire; 2000RPM. Later on a burst limiter and rate of fire limiter were added. The rifle would fire at the full 2000 RPM for two round bursts, but a mere 1000 RPM for full auto. The AKB-1 was a far more conventional design, but it still had a BARS. In this design the countermass was nested inside the main bolt carrier, similar to the AEK-971. Not a great deal of information is available about these rifles, but @Hrachya H wrote an article on them which can be read here.
Now that Weaponsman has linked the forum, I guess it's time to post actual content. No more dumb one-liners or jokes about the Turkish government's policy towards Kurds or Sherman burning down Atlanta. For at least five posts. I think that's all I can manage. The internet has been a mixed blessing for gun nuts. On the one hand, it allowed for much freer exchange of information that was previously exclusive to a few experts. The notorious mil-spec chart (no longer up to date) that circulated around ar15.com years ago is probably a big part of the reason that AR-15 manufacturers stepped up their game and started turning out generally excellent products. On the other hand, the internet has been an excellent vector for the spread of nonsense. In my experience, relatively little of the misinformation is maliciously spread; it's mostly the result of people not knowing what the hell they are talking about. In particular, a great deal of nonsense would be ignored if people could just remember high school physics. A lot of mystical, physics-defying rubbish is said about weapons reliability in particular. Reading nothing but internet fora circa the late 2000s, one could easily come away with the impression that the AR-15 is uniquely unreliable thanks to the direct impingement action. This is despite the fact that coating aluminum or steel in a thin layer of carbon powder would actually reduce its coefficient of friction. Actually, the dynamics of automatic weapons are not difficult to understand. An often overlooked metric in the reliability of gas-operated automatic weapons is the mass ratio between the bolt and the bolt carrier. I first became aware of the importance of this ratio when reading a US Army manual on small arms design at Forgotten Weapons. Just to be clear, this ratio is only important in the way I'm describing in gas-operated, some recoil-operated and inertia-operated weapons. The dynamics for retarded blowback weapons, like the H&K G3, are quite different. In a gas-operated weapon, there is a bolt that is locked rigidly either to the barrel or to the receiver at the moment of firing. This contains the pressure of the cartridge firing (which is alarmingly high). The projectile is pushed down the bore. As soon as it passes the gas port, some of the gas begins pushing the bolt carrier to the rear. The work done on the bolt carrier by the propellant gas from the gas port is the only energy that the bolt carrier will have to complete the cycle of operations. This means that the total work required to: unlock the bolt pick up the bolt extract the spent case cock the hammer compress the return spring operate the belt mechanism (if it's belt fed) cannot exceed the amount of energy that is initially fed to the bolt carrier. Ideally, the bolt carrier will have some excess reserve of energy so that it can complete the cycle of operations even if the gas port is slightly clogged, the ammunition is slightly under-loaded, or the receiver is dirty (et cetera). However, if the bolt carrier has too great a reserve of kinetic energy, it will still be travelling rapidly when it reaches the end of its travel, and then it will violently jerk to a halt or possibly even bounce off of the rear of the receiver. This increases wear on the weapon, and in a shoulder fired weapon can cause the sights to jerk off target. There are two ways to increase the energy capacity of the bolt carrier; make it go faster (in gas-operated weapons, this is done by the simple expedient of enlarging the gas port), or make it heavier. There are practical limits on how fast the bolt carrier can reciprocate; according to Brassey's, a good rule of thumb is 15 m/s is the maximum practical velocity of the bolt carrier. Above this velocity things start to break. Cases can be torn apart instead of cleanly extracted from the firing chamber (the Soviet SHKAS fast-firing aircraft machine gun required special high-quality ammunition with extra-strong cartridge cases), springs lose their strength in fewer cycles, and the lifespan of the moving parts is reduced due to increased fatigue. Making the bolt carrier heavier also has practical limitations; the weapon becomes heavier not just from the heavier bolt carrier, but also from the larger receiver needed to enclose it. In shoulder-fired weapons the sights will be thrown off target by the porpoising motion of the reciprocating center of mass. The act of picking up the bolt after it is unlocked is of particular interest, because it can absorb a great deal of the energy from the bolt carrier. The bolt is picked up and accelerated by the bolt carrier, after which they are stationary relative to each other. This means that there is an inelastic collision between the bolt carrier and the bolt, and in an inelastic collision kinetic energy is not conserved even though momentum is. The initial kinetic energy of the carrier is 1/2MV^2. The initial and final momentum of the system will be the velocity of all moving mass times that mass. This works out to a reduction in kinetic energy after the pickup of the bolt. For instance, if the bolt and bolt carrier have equal mass, after bolt pickup the velocity of the bolt carrier group will be half of what it was before and the mass double what it was before. That works out to half the kinetic energy of the bolt before pickup. Generally the equation for the remaining energy after bolt pickup will be: E2=E1*(1+1/R)-1 Where E2 is the bolt carrier group kinetic energy after bolt pickup, E1 is the bolt carrier kinetic energy before pickup, and R is the ratio of mass between the bolt carrier and the bolt. If anyone is terribly curious I can show the derivation of this. According to the US Army Ordnance small arms design manual hosted at Forgotten Weapons, designers should shoot for a mass ratio of 3 or better, which would translate to 75% energy retention after bolt pickup. More is better, but there are strong diminishing returns here; a weapon with a mass ratio of 2 will have 66% remaining kinetic energy after bolt pickup, but a design with a mass ratio of 4 will have 80% conservation. As the mass ratio gets larger and larger, the percentage of bolt carrier kinetic energy after bolt pickup will approach 100% asymptotically. Again, to keep designs lightweight, the ideal way to achieve this would be to make the bolt as light as possible. However, in a locked breech weapon in 5.56 NATO, the bolt has to withstand peak chamber forces of 22.7 KN (or 5,100 lbs if you're using haram infidel non-SI units). It therefore has to be fairly robustly constructed, and making a lightweight bolt that is still safe over the operating life of the weapon is not an easy engineering task. When determining the ratio for actual weapons, it's important to define exactly what masses are involved. What we are looking at is the ratio of the moving mass that is accelerated by gas that will have kinetic energy available to accelerate the stationary mass. Take a look at this slow-motion video of an FAL firing: The piston follows the bolt carrier back far enough that its energy is available for bolt pickup, even though it's a separate piece from the carrier and doesn't translate through the entire distance that the bolt carrier does during cycling. So, for the purposes of determining bolt carrier to bolt mass ratio, the piston of the FAL counts as bolt carrier mass. In an AR-15, the firing pin rides in the bolt carrier, is pressed forward against the bolt carrier during the initial acceleration, and therefore counts towards the mass of the bolt carrier group. However, the buffer is not rigidly attached to the bolt carrier, so its kinetic energy cannot contribute towards bolt pickup. Therefore it does not get counted. Obviously any extractors, cam pins, and miscellaneous other parts get counted toward the part they ride with. Strictly speaking some portion of the mass of the return spring should be counted (and Chinn's excellent The Machine Gun has equations for calculating the impact of spring mass on reciprocating parts dynamics), but I was too lazy to include this because it's really, really small. So, let's see how well actual designs do: Semi-auto TAR-21 carrier group mass: .623 kg bolt mass: .055 kg Carrier to bolt mass ratio: ~10:1 Notes: It is impossible to remove the return spring from the bolt carrier of the semi-auto TAR-21 for some arcane reason, so I had to guesstimate on this one. However, whatever the exact number, the TAR-21 has an exceptionally high bolt carrier to bolt mass ratio, and that is one of the (few) outstanding features of the design. Using a stationary cam pin that is located in the bolt carrier that acts upon stationary cam grooves in the bolt is one way that the design improves the mass ratio, and this unusual feature is worth emulating in other designs. Sadly, the design as a whole is not as elegant and mass-efficient as the bolt carrier group is. SIG-551A1 carrier group mass: .505 kg bolt mass: .093 kg Carrier to bolt mass ratio: 4.4 Notes: Disappointingly low, but still above the magic 3:1 figure. The SIG rifles show one of the advantages of a reciprocating charging handle; the mass of the charging handle is adding to the kinetic energy of the bolt carrier, rather than being deadweight. The bolt on the SIG rifles is fairly massive, in part because the firing pin is located in the bolt rather than the bolt carrier. 7.62x39mm AK carrier group mass: .505 kg Bolt mass: .080 kg Carrier to bolt mass ratio: 5.3 Notes: The charging handle and piston are one piece with the bolt carrier on an AK, which helps improve the mass ratio. The bolt is fairly massive, in part because the firing pin is located in the bolt. Semi-Auto SCAR-H carrier group mass: .713 kg Bolt mass .066 kg carrier to bolt mass ratio: 9.8 Notes: In the AR-18 derived bolt carrier designs the cam pin is rigidly attached to the bolt, and therefore increases its mass. Also, the bolt must be sturdy enough to withstand the stress of 7.62 NATO ammunition. Despite this, the SCAR-H manages a monstrous 9.8 mass ratio in a rifle that's still reasonably light for an automatic 7.62 NATO weapon.