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Operating Principles of Firearms

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From some posting on ww2incolor designed to educate the resident walt and to make him look like an under-informed fool. Not exhaustive, so please add more

SMGs in general work like this

The receiver of an SMG is basically a metal tube. It might not have a round section like the STEN, but it's a tube nonetheless. Attached to this tube are a magazine housing, a butt, a barrel, and a trigger mechanism.

Inside the tube is a spring and the breech block (sometimes called the bolt). The spring is generally behind the breech block and serves to push it forward. The breech block holds the extractor and (normally) a fixed firing pin. In the STEN, this is machined into the face of the breech block.

When firing an SMG, the breech block is pulled to the rear, where it is held by the sear (part of the trigger mechanism). A magazine, if it has not already been inserted, can be inserted now.

On pulling the trigger, the breech block is released. It runs forward freely for a distance, before contacting the top round in the magazine. It pushes this into the chamber, and just before it would come to rest, the fixed firing pin ignites the primer of the chambered cartridge. This is called "advanced primer ignition" and allows you to reduce the weight of the breech block and/or the strength of the spring. The bullet is now accelerating up the barrel, and the pressure is pushing the cartridge case back against the breech block, which does not lock. This pressure forces the breech block to stop moving forwards, and then to accelerate backwards. By the time it is moving backwards with enough velocity, the bullet is long gone.

The momentum gained by the breech block carries it to the rear, where it ejects the spent case. As it reaches the sear, if the trigger is still pulled the process repeats, otherwise the breech block is held by the sear.

No hammer, no lock, almost nothing.

The FAMAS works like this:

The FAMAS uses Lever-delayed blowback utilizes leverage to delay the opening of the breech. When the cartridge pushes against the bolt face, the lever moves the bolt carrier rearward at an accelerated rate relative to the light bolt. This leverage significantly increases resistance and slows the movement of the lightweight bolt. John Pedersen patented the first known design for a lever-delay system. The mechanism was adapted by Hungarian arms designer Pál Király (a.k.a. Paul de Kiraly) in the 1930s and first used in the Danuvia 43M submachine gun. Other weapons to use this system are the TKB-517/2B-A-40 assault rifles, the AVB-7.62 battle rifle, the San Cristobal .30 carbine, the FAMAS, the BSM/9 M1 and FNAB-43 submachine guns, the Hogue Avenger and B76 pistols, the Sterling 7.62 and AA-52 machine guns.

The M1 Carbine works like this:

The bolt is pretty much a scaled down Garand bolt. The difference is in the operating mechanism. Because the .30 carbine cartridge is significantly lower pressure than the .30-06, the gas take off needs to be closer to the breech. Williams had the insight to realise that a long-stroke at lower pressure was not necessary for the piston, but that you could use a very short stroke at higher pressure coupled with a heavy operating rod.

Thus, you insert the magazine, pull back the charging handle, release, and pull the trigger. Hammer falls, etc etc etc, bullet passes the transverse gas hole, and some gas is taken off & enters the gas cylinder. The gas cylinder moves back very fast about 1/10", where it is stopped by a ring. As it moves back, it pushes the operating rod, giving it momentum. This then causes the bolt to move backwards, and once it has reached its rearmost point, it is returned to battery by the mainspring which is situated under the barrel, acting on the operating rod. As the operating rod comes to its foremost position, it pushes the piston back into place, venting any remaining gas back into the barrel.

Again, the amount of gas taken off is very small compared to the amount present, and thus results in no meaningful loss of muzzle velocity.

Colt M1911 pistol

The barrel and slide are locked together by lugs machined into the top of the barrel at its breech which mate into corresponding recesses in the underside of the slide. On firing, the bullet accelerates down the barrel, where it exits the muzzle. The recoil of the barrel and slide causes them to move back together. They remain locked for a short distance (to allow the pressure to drop), and then a swinging link lowers the back end of the barrel and then arrests its movement, unlocking the two and transferring the momentum to the slide. Slide moves back, ejects case, mainspring brings it forward & chambers the next one. When the slide hits the barrel the swinging link causes it to rise & re-lock. No reduction in muzzle velocity from the self-loading mechanism.

Recoil operated rifles (e.g. Johnson) and MGs (e.g. Maxim, Vickers, MG34, MG42 [although the latter 3 are gas-assisted by muzzle attachments]) work on similar lines - the barrel and breech block recoil together a short distance (normally, except in some pants designs like the Chauchat where they move to the rear together), whereupon the barrel is stopped, the breech block is unlocked, and the breech block carries on to the rear under the momentum it has gained. No meaningful reduction in muzzle velocity is caused by this (if you want to be really anally retentive, the muzzle velocity is reduced by the speed of the barrel moving rearwards, although this speed is so small when compared to the velocity of the bullet as to have no meaningful effect).

9mm Browning GP35 "Hi-Power"

The Browning GP35 is a 9mm Parabellum development of the Colt 1911. The method of dropping the barrel is simplified, with a cam rather than a swinging link. A double-stack magazine was introduced with it, giving the Browning a capacity of 13 rounds (cf. 8 rounds in the Luger & P38).


Cylinder butts against the barrel, but there is necessarily some gap. Cartridge is fired, bullet leaves cylinder & enters the barrel via the forcing cone. Gas is lost through the flash gap, where it does no useful work. Shotgun News did some experiments concerning flash gaps using the M1895 Nagant revolver, which uses an elongated cartridge case to seal the flash gap off. By comparing the velocities of normal cartridges with those that had been cut back so that they would not seal, they found that the flash gap accounted for 100 or so feet/second of muzzle velocity (IIRC, it's a long time since I read the article - it wasn't as much as had been expected).

Walther PP / Walther PPK

These pistols work on the same principle as all straight-blowback pistols: the barrel is fixed, and the slide is held in battery with nothing more than a spring - the barrel and slide are not locked together. On firing, the pressure from the gas in the cartridge case acts on the breech face, and simply blows the slide back. Most straight-blowback pistols are designed as pocket pistols, and are therefore very light. Due to this, and due to the principle of operation, the recoil on this type of pistol can be quite nasty - not only does the bullet leaving the barrel contribute to the recoil, but also the fast-moving slide hitting the stop. The PPK in 9mm Kurz (.380ACP) is not very pleasant to fire, although the 7.65mm Browning (.32ACP) is OK, although better suited to disposing of hamsters and small kittens than charging Russkie commissars. The K in PPK is variously stated as meaning "Kurz" [short, since it is a shortened PP] or "kriminal" [because it was designed for under-cover cops], although the 1st explanation jives better with previous and subsequent use of the letter "k" in German weapon nomenclature (e.g. Mauser Kar98k, H&K MP5k, H&K G3k, H&K G36k, Walther MPK). Walther did make a straight-blowback pistol in 9mm Para called the MP (Militair Pistole), but the spring had to be so strong that it was difficult to operate, and was thus unsuitable for service.

Webley-Scott .455

A peculiar British design, adopted by the Royal Navy. The barrel is connected to the frame by two pairs of diagonal cams. In its uppermost position it locks into the slide. On firing, the barrel and slide recoil, and the barrel drops in a parallel fashion, due to the diagonal cams. This unlocks the slide, which continues to the rear. The slide then returns to battery, being powered by a pair of leaf-springs under the grips.

Other operating principles:

The Bang principle.

John Browning's first patent for a self-loading rifle concerns a modified Winchester lever-action. As already stated above, he noticed that the muzzle blast from a firearm made long grass move significantly. He thus modified a Winchester by placing a small cupped plate with a hole in the middle on a lever over the muzzle, with an operating rod extending to the trigger guard (which forms part of the lever). Several springs were provided to return the lever to battery. On firing, the bullet passes through the hole in the plate, and the gas leaving the muzzle pushes it forwards. Since it is on a lever, it hinges forwards and down, and thus pulls the operating lever forwards.

This in itself was not a practical weapon, but more a development concept that allowed Browning to claim a patent for the gas operation of a firearm (I've seen this patent, and if anyone's interested, PM me & I'll find the number when I'm in work on Monday)

Browning then developed this further for a machine gun made using the same principle (which was prototyped), and then further for a machine gun working by blowing vanes around with the muzzle blast (patented but never adopted), and then hit on the idea of the transverse gas hole in the barrel and took the muzzle blast idea no further.

A Dane by the name of Bang (ironically) did take the idea further. He used a movable sleeve over the muzzle, with the end possessing a hole just large enough for the bullet to pass through. When the rifle was ready to fire, there was an air gap between the end of the barrel and the end of the sleeve (formed by making the end of the sleeve conical), to act as a gas trap. On firing, the sleeve is blown forwards by the muzzle pressure trapped between the barrel and the sleeve. The sleeve is connected to an operating rod, which terminates in a lever, one end of the lever being connected to the op-rod, the other end resting on the bolt carrier. Thus, when the sleeve is blown forwards, the end of the lever pushes the bolt carrier backwards over a short distance with great velocity (i.e. short-stroke).

Wierd, German things

The German military initially had a design constraint on self-loading and automatic weapons, that the barrel was not to be tampered with. They believed, falsely, that this would weaken it. This is one of the reasons why the MG34 and MG42 use a gas-assisted recoil operation, using a muzzle attachment to force muzzle gases to act on the front of the barrel to help push it backwards to increase its recoil.

Thus, when a request for designs of a self-loading rifle was produced, designers were left with the prospect of recoil operation (which results in much higher felt recoil, is complicated, and requires many precision-machined surfaces), or, slightly similar to the Bang principle, but different, an annular piston around the barrel under a Bang-like (although stationary) muzzle cap. There was also, btw, an auxiliary request that the rifles must have no externally moving surfaces and that they must be suitable for use as normal bolt-action repeaters should the self-loading mechanism fail.

This resulted in two rifles, both working on the silly muzzle attachment principle: the G41(M) and the G41(W). M stands for Mauser, and W for Walther:

Battle pics of G41(M) and G41(W) G41

The G41(W) - note the bulbous muzzle attachment containing the annular piston Muzzle

The G41(M) fulfilled the specification perfectly - it even possessed a bolt handle that could be worked rather like the Mauser bolt-actions. However, it was unreliable in combat. The G41(W) did not completely fill the specification, since the bolt carrier moves externally, and it can only marginally be used as a straight-pull bolt action should the self-loading mechanism fail, but was significantly more reliable and simpler to produce.

There is a reason that the Bang amd other muzzle attachments never had any real success - they fouls quickly then jam, and are very sensitive to dirt.

By this time, the Germans were capturing significant numbers of Tokarev SVT38s and SVT40s, which were often used by German troops as substitutes for their own weapons, and they were well-liked. The Russian conscript infantry didn't like them, since they required lots of cleaning and were more complicated to use than a bolt-action.

The Germans then looked at the success of the SVTs and realised that it did not weaken the barrel by boring the transverse hole to tap gas, and the G41(W) was quickly re-designed to use the Tokarev gas system, the bolt and carrier remaining essentially the same. This became the G43. The G41(M) was a dead-end and was not developed further. In the meantime the FG42 and MKb42 had been designed, which also used a transverse gas hole. The prejudice against it was thus laid to rest forever.

G43 rifle - note that the receiver is identical to the G41(W), and the magazine is not attached G43

The Tokarev/G43 gas system involves taking gas from a port in the barrel under a gas block quite a distance back from the muzzle, and directing it into a gas tube, which projects from the gas block towards the action. Over the gas tube is a gas cylinder which is a sliding fit and possesses vent holes to limit its motion under gas pressure. This sliding gas cylinder is attached to the operating rod, which contacts the bolt carrier. This whole gas system is suspiciously similar to the AR18/SA80 system...

On firing, the bullet accelerates along the barrel and passes the gas port. A small amount of gas is directed into the gas plug, via the gas block. This acts on the gas cylinder, pushing it to the rear until the vent holes are exposed by the gas plug, and then the excess gas is vented. The bolt and carrier continue to the rear, whilst the op-rod spring returns the op-rod and gas cylinder to their rest position. The mainspring returns the bolt to battery, where it is locked by means of a pair of flaps (the Tokarev is locked by a tipping block).