8 July 19
Extraction:
With military autoloading rifles, pistols, and machine-guns, there are several methods of accomplishing the autoloading function, and all “work,” to one degree or another, and of course, all have issues, successful case extraction being the most critical.
Extraction always involves two sequential phases:
SIE (slow, initial extraction), followed by case withdrawal.
In the SIE phase, the case (expanded as far as chamber dimensions permit during firing) shrinks back to original dimensions when the bullet finally exits the muzzle and pressure drops to zero.
The case thus “breaks-loose” from adhesion to chamber walls and is then free to move backwards under its own momentum, combined with persuasion from the extractor.
1. The “Delayed Blowback” system is found on both pistols and rifles.
Barrel is fixed. Separation of the barrel and bolt/slide upon discharge is “delayed” by several nanoseconds in order to allow the accelerating bullet to clear the muzzle and pressure in the barrel to then instantly drop to zero.
This critical delay is accomplished via a miniature gas-piston (H&K P7), via rollers (H&K/PTR 91), via levers (French FAMAS), via mechanical advantage (Pederson Rifle), or just a pistol slide or SMG bolt with a lot of weight (stationary momentum.)
These guns all boast high accuracy, as the barrel never moves. Reliability is also very good.
However, of all current autoloading systems, bolt/slide rearward acceleration on delayed-blowback guns is the highest, which invariably brings about extraction problems, along with sharp recoil!
With high bolt/slide rearward acceleration (upon firing), still incomplete SIE sometimes causes fired cases to “stick” in the chamber, despite tugging on the fired case’s extraction-groove by the weapon’s extractor.
Subsequently, under aggressive extraction, fired cases pull apart (the rear half ejects, the front half stays in the chamber), or the extractor pulls-through the cartridge’s extractor ring, leaving the entire fired case in the chamber.
Or, the extractor claw breaks (robust extractors break less often than do small extractors found on multi-lug bolts)!
Either way, the shooter is faced with a stoppage that is not easily remedied in the field.
Some will point-out that the extractor on delayed blowback guns (particularly pistols) is all but unnecessary, since the case literally pushes the bolt backward. However, the extractor on these guns does perform the important function of holding the case in the appropriate position for smooth ejection.
There are five ways this “stuck-case” issue is, and has been in the past, addressed, with varying degrees of success:
a. Cartridges coated at the factory with a “mold-release.” This was the solution selected by John Pederson for his rifle (1930s). The waxy coating was applied at the factory, and fired cases then extracted smoothly.
Upon firing, the coating vaporized and (mostly) ejected from the muzzle, right behind the bullet.
However, the whole idea (along with the Pederson Rifle itself) was rejected by the War Department, over worries about the extra step in ammunition manufacture, and residue build-up.
Some steel-cased ammunition, currently manufactured in Eastern Europe, comes from the factory with a “lacquer” coating that, once more, functions as a “mold release.” Steel is far less suitable for rifle cartridge cases than is brass, owing to its relative lack of elasticity. But, a world-wide copper shortage has necessitated its use in some places.
b. Cartridges receive a “wipe” or “spritz” of oil just prior to chambering.
This system was used on several heavy machine guns (mostly water-cooled) during the last century. It largely worked, but complicated the lives of gun crews and added yet more weight and moving parts to an already ponderous, maintenance-heavy system.
And, in an age of “maneuver warfare,” water-cooled machine guns are obsolete! They’re too heavy and take too long to move and set-up.
c. “Blow-Past” rifling.
The Steyr GB Pistol (out of production since 1988) is the only gun I know of employing this system.
The Steyr GB was a gas-delayed blowback pistol with rifling grooves on the inside of the barrel cut so deep that the bullet did not form a perfect seal. Thus, some percentage of expanding gas actually blew past the bullet as it was propagated down the barrel.
The effect was to round-off the pressure spike associated with launch and thus reduce the slide’s rearward acceleration, and in turn, reduce extraction problems.
It successfully addressed the extraction issue, but at the sacrifice of muzzle velocity.
d. Grooved chamber.
This is the system H&K, PTR, and a few others currently employ on their roller-delayed blowback rifles, also the gas-delayed H&K P7 pistol (now out of production)
Grooves are cut into the inside of the chamber, which allow high-pressure gas to seep back along the case wall as soon as the bullet leaves the case mouth and starts down the barrel. Expanding gas within these grooves breaks-loose the fired case and makes subsequent extraction and ejection relatively smooth.
e. Stationary momentum of the slide/bolt itself.
This system is only suitable for pistols and pistol-cartridge-firing longarms (SMGs, et al). It necessitates a heavy slide/bolt. SMGs using this system has been popular with armies in the past century, because they were easy and cheap to manufacture.
Current pistols using this system are also cheap, but typically heavy, awkward clunks, and thus not particularly suitable to concealed carry.
They enjoy scant popularity today, at least among sophisticated consumers.
2. Recoil operation (“short” and “long”).
In this system, both barrel and slide/bolt move backward while locked together immediately after discharge and as long as the bullet it still in the barrel.
This short rearward movement with both components locked together provides the necessary dwell-time for the bullet to exit the muzzle, and barrel pressure to then drop to zero.
As the slide/bolt continue to the rear, they unlock, separate, and extraction and ejection can then take place normally.
Most modern pistols employ this “short recoil” system.
With the “long recoil” system, barrel and bolt remain locked together until the end of the bolt’s rearward travel.
Then, the barrel unlocks and moves back forward, separate from the bolt, and the bolt finally moves forward too, picking up a fresh round in the process.
This ‘long recoil” system (CSRG, or “Chauchat” from WWI) is virtually unknown today, with the exception of some shotguns
Bolt/slide rearward acceleration with either “short” or “long” recoil operation is far less aggressive than is the case with the delayed blowback system, and thus chamber-grooving is seldom necessary.
3. Gas operation (“short” and “long”)
Gentlest of all rearward bolt acceleration is found with gas-operated rifles, and accordingly they enjoy, for the most part, the fewest extraction problems.
A few gas-piston pistols have been manufactured (the now out-of-production Widely, for example), but none have ever enjoyed much popularity.
An exception, at least semantically, of course are a modern generation of “long-range pistols,” which are basically short-barreled, gas-operated rifles, combined with “arm-braces” that function like conventional stocks.
They’re labeled “pistols” by their manufacturers, for political/commercial reasons.
Gas-operated rifles are currently, by far, the most common in use for military and other serious purposes.
All employ a “gas-tap,” or hole, somewhere in the barrel, where high-pressure gas is diverted into an expansion-chamber. A piston at the rear of the expansion chamber is then pushed backward against spring-pressure to provide mechanical motion necessary to complete the rifle’s cycle of operation.
The expansion chamber is pressurized for only the few nanoseconds between when the bullet passes over the gas-tap hole and when it subsequently exits the muzzle.
When the piston has moved to the rear less than a centimeter, it uncovers a vent-hole, which vents all remaining gas to the outside, and all pressure then instantly drops to zero. This happens about the same instant the bullet is exiting the muzzle and pressure is dropping to zero anyway.
So, the system is “pressurized” for only a a few nanoseconds, but that provides sufficient momentum to mechanical parts to complete the cycle of operation, as noted above.
With some rifles, the expansion chamber is “adjustable,” so that some percentage of high-pressure gas can be vented instantly, thus “regulating” the acceleration of the gas-piston.
With the “long” gas system (found of the AK, RA/XCR), pistol, op-rod, and bolt-carrier are all one piece and all move together, rearward and then back forward.
With the more common “short” gas system (FAL, POF Revolution), the piston/op-rod, and bolt-carrier are separate pieces. The effect is the same, but with less movement of parts.
POF’s Revolution Rifle employs this “short” gas system, but to insure flawless extraction, chambers are also grooved (the “E2 System”). POF is the only one doing this that I know of.
The “Stoner System,” sometimes called “direct gas impingement” is actually just a variation.
With the Stoner System (M4, et al), the expansion chamber is actually inside the bolt-carrier. The bolt-carrier itself becomes the “piston,” and high-pressure gas is delivered to it via a long “gas-tube” all the way from the gas-tap hole in the barrel. The op-rod is thus eliminated.
All three variations are satisfactory, and all has weaknesses. However, extraction is usually not one of them.
All can be “under-gassed,” and all can be “over-gassed!”
In the former case, symptoms include sluggish operation and ultimately incomplete cycling.
In the latter, symptoms include “bolt over-ride,” which results in the forward-moving bolt hitting the middle of the next round, instead of its case head, or the bolt over-riding the next round altogether and subsequently chambering thin air!
The “Goldilocks Zone” is always somewhere between these two extremes!
GPMGs, like the IMI Negev, which can be fed from either a disintegrating belt, or a conventional rifle magazine, come with user-operated gas-adjustment, because much energy is consumed advancing the belt through the feeding system, and thus much gas is needed when firing from this mode.
By contrast, when the Negev is fed via a magazine (which consumes far less energy), gas must be dialed-back to avoid over-gassing symptoms described above.
The foregoing is intended as a ‘Primer” on autoloading weapons. Many more details could be included, of course, but I hope it satisfactorily answers some of the common questions asked by students.
What it is our honor and privilege to use today is the product of over a century and a half of ingenious design, experimentation, and bitter field experience.
It is our duty to all those who sacrificed so much creating and refining these systems to learn to use them to their maximum potential!
/John
