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Sturgeon's House

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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:




Where E2 is the bolt carrier group kinetic energy after bolt pickup, Eis 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.
carrier group mass:
.505 kg
bolt mass:
.093 kg
Carrier to bolt mass ratio:

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:
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:

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.

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  • 3 weeks later...



Carrier group mass:

.444 kilograms


Bolt mass:


.106 kilograms


carrier to bolt mass ratio:




Notes:  The striker/linear hammer thingie and the gas piston were included in the mass of the carrier group, as manually fiddling with the piston and high speed video show that the piston reciprocates quite far enough to assist in unlocking.  The mass of the locking piece was, of course, included in the mass of the bolt group.


The bolt carrier itself is only .348 kilograms, and is an extremely complex piece of machining.

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  • 3 weeks later...

Alright, here are the mass ratios and unloaded weights for the 25 guns we looked at today. We got maybe 2/3s of the way through Alex's gas-operated rifle collection.
Weight: 3.039 kg
Magazine Weight: 0.082 kg (20 rd)
Bolt Mass: 0.050 kg
Carrier Mass: 0.302 kg
Oprod Mass: 0.100 kg
Mass Ratio: 6.04
Mass Ratio (Unlocking): 8.04
Benelli MR-1
Weight: 3.640 kg
Magazine Weight: 0.122 kg (10 rd)
Bolt Mass: 0.076 kg
Carrier Mass: 0.258 kg
Oprod Mass: 0.092 kg
Mass Ratio: 3.39
Mass Ratio (Unlocking): 4.61
Bushmaster ACR
Weight: 3.766 kg
Bolt Mass: 0.050 kg
Carrier Mass: 0.386 kg
Oprod Mass: 0.064 kg
Mass Ratio: 7.72
Mass Ratio (Unlocking): 9.00
Bushmaster M17S
Weight: 3.666 kg
Bolt Mass: Unknown, assembly does not come apart
Carrier Mass: Unknown, assembly does not come apart
Oprod Mass: Unknown, assembly does not come apart
Bolt Group Mass: 0.540 kg
Colt 6920
Weight: 3.054 kg (w/ sling)
Bolt Mass: 0.057 kg (measured in ounces, oops)
Carrier Mass: 0.267 kg (same)
Mass Ratio: 4.68
Daewoo K1A1 16" Barrel
Weight: 2.962 kg
Bolt Mass: 0.050 kg
Carrier Mass: 0.294 kg
Mass Ratio: 5.88
Faxon ARAK-21
Weight: 3.544 kg
Bolt Mass: 0.056 kg
Carrier Mass: 0.312 kg
Mass Ratio: 5.57
Weight: 3.72 kg
Bolt Mass: 0.072 kg
Carrier Mass: 0.556 kg
Mass Ratio: 7.72
FN FS2000
Weight: 3.694 (w/ tuna optic)
Bolt Mass: 0.054 kg
Carrier Mass: 0.420 kg
Mass Ratio: 7.78
Weight: 3.214 kg
Bolt Mass: 0.072 kg
Carrier Mass: 0.512 kg
Mass Ratio: 7.11
Weight: N/A, forgot to weigh
Bolt Mass: 0.072 kg
Carrier Mass: 0.634 kg
Mass Ratio: 8.81
Galil ARM
Weight: 4.404 kg
Bolt Mass: 0.080 kg
Carrier Mass: 0.436
Mass Ratio: 5.45
Gwinn Firearms Bushmaster
Weight: 3.426 kg
Bolt Mass: 0.056 kg
Carrier Mass: 0.660 kg
Mass Ratio: 11.79
Weight: 2.958 kg
Bolt Mass: 0.054 kg
Carrier Mass: 0.494 kg
Oprod Mass: 0.062 kg
Mass Ratio: 9.15
Mass Ratio (Unlocking): 10.30
Weight: 2.828 kg
Bolt Mass: 0.058 kg
Carrier Mass: 0.272 kg
Oprod Mass: 0.116 kg
Mass Ratio: 4.69
Mass Ratio (Unlocking): 6.69
M1 Garand
Weight: 4.790 kg (w/ leather sling)
Bolt Mass: 0.158 kg
Carrier Mass: 0.276 kg
Mass Ratio: 1.75
Weight: 4.153 kg
Bolt Mass: 0.188 kg
Carrier Mass: 0.242 kg
Mass Ratio: 1.29
Masterpiece Arm MPAR
Weight: 4.038 kg
Bolt Mass: Unknown, assembly does not come apart
Carrier Mass: Unknown, assembly does not come apart
Oprod Mass: Unknwon, assembly does not come apart
Bolt Group Mass: 0.546 kg
Robinson Arms M96
Weight: 3.994 kg
Bolt Mass: 0.060 kg
Carrier Mass: 0.472 kg
Mass Ratio: 7.87
Ruger Mini-14
Weight: 3.064 kg
Bolt Mass: 0.080 kg
Carrier Mass: 0.370 kg
Mass Ratio: 4.625
SIG 556
Weight: 3.974 kg
Bolt Mass: 0.092 kg
Carrier Mass: 0.398 kg
Mass Ratio: 4.33
Steyr AUG A3
Weight: Slightly less than 3.600 kg (rifle had optic, we subtracted weight of identical optic with slightly heavier mount)
Bolt Mass: 0.056 kg
Carrier Mass: 0.452 kg
Mass Ratio: 8.07
Weight: 4.552 kg
Magazine Weight: 0.398 kg (blued), 0.392 kg (parked)
Bolt Mass: 0.188 kg
Carrier Mass: 0.466 kg
Mass Ratio: 2.48
Weight: 3.600 kg
Bolt Mass: 0.054 kg
Carrier Mass: 0.418 kg
Mass Ratio: 7.74
Valmet M76
Weight: 3.614 kg
Magazine weight: 0.282 kg
Bolt Mass: 0.082 kg
Carrier Mass: 0.430 kg
Mass Ratio: 5.24



Here are the image files, if anyone wants them.

Takeaways: First, sometimes the results really surprised me. I expected the Faxon ARAK-21 and the Bushmaster ACR to have mediocre mass ratios, less than an AR-15, but they actually had higher ratios. The Gwinn Firearms Bushmaster was insane, with the highest ratio of the day. Second, Stoner-Johnson bolts (a la AR-15) are very light, approximately 2/3s the weight of a comparable Kalashnikov-style or Garand bolt. The SCAR (both 16 and 17) is evidence of this, with its bolt being basically a 7.62 version of a G36 bolt. Finally, having a high mass ratio appears to be a poor indicator of reliability. A higher mass ratio is surely better, but so many things are involved in system reliability for a firearm that mass ratio seems almost secondary, even though it's a core design consideration. The MAS 49, for example, has the worst ratio on the list, even though it's reputedly a quite reliable firearm. In contrast, the ARAK-21 had a mass ratio higher even than an AK, but they are total stinkers. Mass ratio I think is an important consideration for small arms design, but it's almost meaningless to the consumer, and certainly shouldn't be a point of rejection or acceptance for a civilian buyer.

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I did many more rifles and submachine guns today, but only 8 more for which there were relevant mass ratios. They were:


M1 Carbine
Weight: 2.080 kg
Magazine Weight: 0.078 kg
Bolt Mass: 0.096 kg
Carrier Mass: 0.224 kg
Oprod Mass: Tappet mass unknown
Mass Ratio: 2.33
Mass Ratio (Unlocking): Unknown
Springfeld M1A
Weight: 4.432 kg
Magazine Weight: 0.232 kg
Bolt Mass: 0.146 kg
Carrier Mass: 0.212 kg
Oprod Mass: Unknown
Mass Ratio: 1.45
Mass Ratio (Unlocking): Unknown
Weight: 3.424 kg
Bolt Mass: 0.134 kg
Carrier Mass: 0.236 kg
Oprod 1 Mass: 0.064 kg
Oprod 2 Mass: 0.020 kg
Mass Ratio: 1.76
Mass Ratio (Unlocking): 2.39
Weight: 3.934 kg
Magazine Weight: 0.182 kg
Bolt Mass: 0.102 kg
Carrier Mass: 0.204 kg
Mass Ratio: 2.00
Ljungmann AG m/42
Weight: 4.414 kg
Magazine Weight: 0.258 kg
Bolt Mass: 0.162 kg
Carrier Mass: 0.226 kg
Mass Ratio: 1.40
Saiga 5.45x39 (Converted)
Weight: 3.414 kg
Bolt Mass: 0.072 kg
Carrier Mass: 0.410 kg
Mass Ratio: 5.69
Saiga 7.62x39 (Converted)
Magazine Weight: 0.320 kg
Bolt Mass: 0.072 kg
Carrier Mass: 0.404 kg
Mass Ratio: 5.61
Weight: 4.358 kg
Magazine Weight: 0.240 kg 
Bolt Mass: 0.174 kg
Carrier Mass: 0.294 kg
Oprod Mass: 0.110 kg
Mass Ratio: 1.69
Mass Ratio (Unlocking): 2.32


It's funny to note three things. First, the M1A fares even worse than the M1 Garand. Second, the G1 FAL, the heaviest issue FAL configuration, is in fact lighter than a Springfield M1A (!!! this fact almost makes me doubt the data... But there it is. I will check tomorrow). Third, the SKS is in actuality about a pound lighter than Wikipedia says it is.


There were on hand a K43, Vene FN-49, and a G.41 (M). We measured none of them, except for weight. In the case of the G.41(M) in particular, we did not feel we could field strip it without risking damage to the gun, which is in incredible condition for a weapon of that type. We also had an AR-70 for measure, but it was in pristine condition. Alex nobly pulled the gas tube out, allowing access to the piston, and we measured it... And I forgot to take photos. D'oh! However, I did get the rifle's weight, and its mass ratio should be very AK/SIG-like, so it's not a great loss.

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It's also surprising to learn that, in addition to making the receiver heavier when compared to a rotating bolt, tilt-locking also makes the bolt heavier.  A lot heavier.


Also, WTF is up with the Rasheed?  It's heavier than the SKS.  Egypt had switched to 7.62x39mm by the time they were using them, obviously, but they wanted to make a modified Ljungman?!  Does it look like it shares any parts or production tooling?  What possible advantage could it possess over the SKS?


At least the SKS isn't a derivative of a design notorious for experiencing failures when exposed to sand.  You know, that stuff that Egypt is made out of.

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  • 1 month later...

New thought;


can tilting bolts withstand higher bolt carrier velocities?  On most rotating bolt designs, especially recent ones, there's a big hole through the bolt that, at least in some cases, compromises the strength of the bolt:




But on a tilting bolt design the bolt locks and unlocks after being cammed by beefy metal hooks:




We know from the data collected above that tilting bolts are heavier, and so waste more energy bolt carrier kinetic energy by being unlocked.  But can they afford to be smacked harder, thereby offsetting that inefficiency?

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This was my immediate thought after weighing all of Alex's guns. It's possible that with some cleverness a locking system could withstand higher velocities, but I would caution anyone looking to design something around this, because as we saw with the M1 Garand, one of the potential consequences of this is galling of the locking surfaces. A tilting bolt design is probably susceptible to the same effect.

However, I do think maybe this analysis allows designers to move past religious adherence to the "covet thy mass ratio" maxim and design guns that are just as reliable but potentially somewhat lighter in the operating group. The basic principle still applies; it's not like physics isn't in effect here, but one could perhaps gnaw at the margins of the system by using hardened locking surfaces or other tricks to get away with a lower mass ratio.

Still, now that I have two M1As for review, it's reeeaaaaally clear that the M14 is suffering in this regard, though.

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Oh?  Is there noticeable galling at the bolt camming surfaces or something?


...Are you unfamiliar with the famous M1 bolt galling problem? Here's a bit I wrote for LRIII:


"Extensive harsh environment testing conducted at the beginning of the war of the M1 rifle revealed that if sufficient lubricant were washed away from the operating rod cam surfaces, then the metal surfaces of the cam track and bolt camming lug would gall and stick together, causing a failure to cycle. One of five possible solutions to this problem were investigated: The first was fluting the chamber to ease extraction, the second was to redesign the operating rod and bolt cam angles to slow unlocking, the third was to improve the finish and surface treatment of the bolt and operating rod cam surfaces, the fourth was to use a special heat treatment for the cam surfaces, and the last was to enlarge the gas port diameter to .100″ to give the action additional power. The M1E1 rifle represented the product of the second solution, and featured a new bolt and operating rod with camming surfaces machined at a more generous angle to slow down the unlocking of the bolt.

"Eventually, the solution chosen was the issuance of more water-resistant Lubriplate 130A lubricant, which essentially solved the problem without the need to change any element of production.
"The continuation of investigations into reducing galling of the cam surfaces resulted in the M1E3 rifle, which was the logical conclusion to the M1E1 line of investigation. Instead of simply redesigning the angle of the camming surfaces, the M1E3 introduced a roller cam lug in place of the shaped, fixed cam lug of the original M1 and M1E1. This roller-lug redistributed the friction experienced by the cam surfaces and was perhaps the most successful solution to the galling problem. However, the replacement of the cam lug with a roller came with a downside: The friction-reducing anti-pre-engagement surface of the M1 rifle had to be eliminated in its implementation, but the feeling was that this was well-worth it to fix the galling problem for good. It should also be noted that the roller-lug design as implemented by Springfield Armory did not provide a roller surface between the bolt and receiver, only between the lug and the operating rod."
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