Tuesday, May 29, 2007

Operating Systems

The question was recently asked "What's the difference between blowback and recoil operation?".

If you aren't a gun nut, those terms probably don't figure much into your daily conversation. In fact, you've probably never heard of them. Even if you are a shooter, unless you are into the technology and engineering of guns, you may not know the terms.

What they are, are labels for two of the most common methods of operating a self loading (meaning automatic, or semi automatic) firearm.

In a self loading weapon, the operating system uses the energy of a firing cartridge, to eject the spent casing, load the next round, and re-cock the weapon; ready to fire again with the next pull of the trigger (or if fully automatic, firing again as soon as the cycle completes).

There are three fundamental ways of using the energy of a firing cartridge to cause a weapon to cycle itself (weapons that cycle off of external power, like Gatling guns are not counted here).
  1. Gas Operation
  2. Blowback Operation
  3. Recoil Operation
Gas operated systems tap the pressure of the expanding combustion gasses (through a piston, lever, gas trap, tappet, op-rod or direct impingement) to force the bolt, slide, or other breech mechanism back and cycle the action.

This system has physical complexities relating to regulating gas pressure (which can vary wildly), and timing of the operating cycle; but is less sensitive to firing position, weapon mounting, and cartridge power; as well as being less dependent on finely fitted moving parts; than other self loading systems.

As such, gas operation is the most common operating mechanism for assault rifles and light machine guns; as well as the operating mechanism for a large number of automatic shotguns (many are recoil operated instead). Some pistols also use gas operation, but because of the additional weight and bulk required for a gas system, this is uncommon.

The most prominent examples of gas operation, in its major variants (in order of mechanical simplicity) are:
  1. The AR platform rifle (AR-10 and 15, M16, and variants) which uses direct gas impingement; where the gas from the barrel acts directly on the bolt carrier.

    This system has the most inherent accuracy of any gas operated system; but is also sensitive to fouling, and is the most difficult system to time properly.

  2. The AK 47 and variants, which uses a short stroke linked piston op-rod design.

    In the AK system, the op-rod, piston, and bolt are mechanically fixed to each other and thus travel in a long stroke; but the gas only works on the piston for a short distance before it is vented, so it is a short stroke gas system.

    This system is mechanically very simple and reliable, but is detrimental to accuracy; because the gas system is physically linked to the bolt, and to the barrel; which induces unpredictable vibration into the barrel.

  3. The M1 carbine, which uses short stroke tappet system to cycle the action.

    In a tappet system the gas piston and tappet, which may or may not be physically linked (or even a single part), move a short distance very rapidly under gas pressure. After the gas pressure is cut off (either through a mechanical limit to the piston stroke, or a gas port cutoff actuated by the pistons stroke) the tappet flies back through inertia imparted it by the gas piston. At this point the tappet may be in contact with the bolt or op-rod; or it may travel a short distance to strike an op rod, or travel further to strike the bolt directly; in either case imparting a great deal of inertia to the op-rod or bolt. The bolt then flies back against spring pressure without any further mechanical connection to the gas system.

    This system is a good compromise of mechanical simplicity, accuracy, and reliability; because it uses the reliable gas piston system which doesn't foul as much or have as sensitive timing as the direct impingement system; but it doesn't mechanically fix the gas system directly to the barrel and bolt; which reduces vibration.

  4. The M1 Garand, which use a long stroke, long rod gas piston system which has a bolt, op-rod, and piston in three separate parts, not mechanically fastened to each other (but which remain physically connected to each other through spring pressure and friction fit). The entire system cycles for the full length of the operation cycle; thus "long stroke".

    This system is somewhat detrimental to accuracy (though not as much as in the gas system of the AK47), and is sensitive to very high or very low gas pressures (either bending the op rod, or short stroking the action); but is otherwise very reliable.
* It's not significant enough to be called a major variant; but the the M14 uses a design that combines elements of the tappet system of the M1 Carbine, and the long rod system of the M1 garand. It uses a short stroke gas piston that cuts the gas pressure off quickly, and an op rod that doesn't maintain continuous contact with the piston.

Blowback operation uses the reaction of the cartridge casing "blowing back" (the equal and opposite reaction to the bullet blowing forward) to push the breech mechanism back and cycle the action. It is mechanically the simplest type of self loading action, because fundamentally the only moving parts are the breech mechanism, and the cartridge case itself (not counting the spring which pushes the breech back into place after ejection). Effectively a blowback operated weapon is in fact a gas operated weapon; with the gas piston in this case being the cartridge casing.

There are two common major variants of the blowback system: Simple Blowback, and Delayed Blowback.

Simple Blowback operation is the most common operating system for pistols firing cartridges with less energy than 9mm parabellum (9x19). Without a delaying or supplemental locking mechanism, breech mechanisms have to be very heavy to handle higher energies; thus making pistols using simple blowback for more powerful cartridges very heavy and bulky. Many submachine guns, and pistol caliber carbines however use the simple blowback mechanism because the extra weight of the blowback bolt isn't a problem.

Delayed blowback is called this, because there is an inbuilt mechanism to delay the breech blowing back, until pressures have dropped to the point where the cartridge can extract safely. Common methods include roller lockers, gas delay, fluted chambers, friction locks (like the Blish lock, which proved ineffective), rotating lugs, or some combination of all of the above.

Delayed Blowback systems are somewhat common in assault rifles, battle rifles, and machine guns. Some pistols and SMGs also use delayed blowback, but this is uncommon.
The most significant examples of simple blowback systems are:
  1. The Makarov pistol (9x18mm), which is by far the most manufactured centerfire blowback pistol, and which uses a textbook example of simple blowback.

  2. The UZI submachine gun; which also uses a textbook simple blowback system; slowing down it's operation for use with high pressure 9mm nato and .45acp loads by using a very long, very heavy telescoping bolt design.
Also, it should be noted that almost all .22 auto pistols use a simple blowback operating system.

The most significant examples of the delayed blowback system are:
  1. The HK P7 pistol, which uses a gas delay mechanism. This system is very rare; only used on a half dozen weapons in modern circulation.

  2. The CETME/HK G3 series of rifles, and MP5 series of submachine guns (all based on the same design), which all use a roller delayed blowback system (the CETME/G3 also uses a fluted chamber to aid in the delay. The fluted chamber increases friction on the cartridge case as it extracts, slowing down the unlocking and blowback).
Delayed blowback systems are relatively uncommon, because they are extremely difficult to manufacture reliably. They require high precision machining, with very tight tolerances; and are very sensitive to differing operating pressures.

Recoil operated systems uses the energy of the recoil of the firing cartridge to cycle the weapon; by locking the breech and barrel together and causing the whole mechanism to be forced back (again an equal and opposite reaction to the bullet being forced forward).

There are two common implementations of this system Short Recoil and Long Recoil.

In the Short Recoil system, the breech and barrel are locked together only for a short time. As they travel back, a mechanism unlocks the breech from the barrel (typically a tilting link, or a sliding cam), which then stops moving; and the the breech continues moving back to it's full cycle length; extracting and ejecting the spent cartridge casing before being forced back into battery by the operating spring, or recoil spring (which may be the same thing).

A variant of the short recoil system uses a hybrid of short recoil and delayed blowback, by keeping the barrel and breech locked together through rotating or camming locking lugs that unlock as the mechanism cycles.

Short recoil is the most common operating system for centerfire pistols more powerful than 9mm Makarov (9x18mm); but is uncommon in most other types of weapons.

The most significant examples of this operating system are:
  1. The Browning M1911 pistol, which uses a tilting barrel connected to the frame with a link pin, and locked to the breech with locking lugs machines into the barrel and slide

  2. The Browning P35 Hi-Power pistol, which also uses a tilting barrel; but which connects the barrel to the frame with a machined cam lug that is an integral part of the barrel; and a cam pin which is fixed to the frame.

  3. The Browning M2 machine gun, the basic heavy machine gun of the armed force of the United States (and many other countries) from it's introduction in 1921 'til the present day.
Almost all modern centerfire handguns in a major caliber use a variant of the operating system in one of these two pistols. Most heavy machine guns use a variant of the operating system of the M2 (or a long recoil mechanism)

*A minor, but significant variant, is the sliding cam system used only by Beretta currently, but introduced by Walther in the P38; the first successful double action auto service pistol. In this system neither the barrel nor slide tilt relative to each other. They remain rigidly locked by a sliding cam separate from both, until that cam is moved out of position by the rearward cycling of the slide. It is significant both because it is the only major caliber centerfire semiautomatic pistol operation system in common use not designed by John Browning; and because the Beretta M9 is the service pistol of our armed forces.

In the long recoil system, the entire barrel and breech mechanism remain locked together to the limit of the cycle; where the bolt is held back, and the barrel is then forced forward by the recoil spring. After the barrel travels forward (it may lock forward all the way first, or it may not), the bolt is then released, ejecting the round (some systems eject the round as soon as the barrel moves forward), stripping a new round, and slamming back into lock with the barrel.

This mechanism is common in machine guns, and automatic shotguns; but uncommon in all other types of weapon.

The most significant example of the long recoil system would be:

  1. The Browning Auto 5 shotgun, which was the first successful semi-automatic shotgun design, and is the longest produced semi-auto shotgun by far at 98 years of regular production.

    Most automatic shotgun designs were based on this same principal until the introduction of the gas operated Remingtion 1100 in 1963.
You'll note, John M. Browning designed most of the significant recoil operated weapons I referenced. Though he did not invent the concept of recoil operation (Hiram Maxim made the first practical recoil operated gun), Browning perfected and popularized it; making it the most common system of operation for centerfire handguns and machine guns today.

Funny enough, Browning also invented the first successful gas operated gun, the Browning model of 1895; which was briefly adopted by the U.S. as a standard machine gun; and the first truly successful blowback semi-automatic pistol, the Colt model 1903.

Based on his experiences developing these (and other) weapons, Browning believed that blowback and gas operation were both inferior to recoil operation, from an engineering and physics standpoint.

From a pure mechanical and physical standpoint he is correct, in that recoil operation is the simplest and most efficient system capable of handling major power cartridges.

Unfortunately, to be reliable, durable, and accurate; recoil operation also requires very good production tolerances, fine machining, and excellent metallurgy. This is why most firearms manufactured in the third world, or former Com-Bloc countries are either simple blowback, or gas operated with a simple piston/rod system.

So what system is best?

Well, it really depends on what type of weapon you're talking about; and what you want to do with it. There isn't really a "best" system, or all guns would use it; but there are better choices for certain applications.

Clearly, gas systems are the most versatile for rifles (not necessarily best, but definitely most versatile); because they can be adapted to run most anything; but they aren't appropriate for most pistols because of their bulk. Also, gas systems are by their nature dirtier and more sensitive to fouling than other operating mechanisms. The most accurate gas system, direct impingement, is also the dirtiest.

Delayed blowback systems are more inherently accurate than gas or recoil operation (both for rifles and pistols, because there is no vibration induced by a gas piston, and no barrel motion in recoil), but in pistols they are more mechanically complex than recoil operation; and in rifles they are limited in the power and range of cartridges they can fire reliably.

Short recoil is certainly best for major caliber pistols; but it isn't necessary for .22s, and in fact the mechanical rigidity of the blowback system improves accuracy (no matter the chambering).

Just some general principles to guide you in figuring out an operating system:

  • Less motion is good
  • Less mass in motion is good
  • Slower motion is good (presuming sufficient energy)
  • More consistent motion is good
  • Fewer moving parts are good
  • Fewer closely fitted parts required is good
  • Better tolerances and tighter clearances are good
Reliability (all of the above 'cept the last, plus):
  • Less sensitive to timing is good
  • Less sensitive to pressure is good
  • Less sensitive to firing position is good
  • Less sensitive to weapon mounting or grip is good
  • Less sensitive to fouling is good
  • Less fouling overall is good
  • Looser clearances required are good
Power and Versatility:
  • Less sensitive to pressure is good
  • Less sensitive to inertia is good
  • Less sensitive to recoil is good
  • Less sensitive to overall power of the cartridge (one way or the other) is good
Unfortunately, you can't have all those things at once. To optimize your choices, you need to look at what your needs are for accuracy, reliability, power, and versatility, and make your compromises.

Best of luck, no-ones come up with the perfect solution yet... Maybe when we have hand held plasma pistols with an instantaneous recharge rate, and unlimited power source...

...'Til then I'm pretty happy with a 1911, an AR, an M14, and my manually operated pump shotgun and revolver.