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Are you interested in firearms, both military and civilian? Did you ever wonder how such instruments worked? Why should I use x caliber over something else? Perhaps you found your grandfather's old rifle and want to know something about it. Even better, perhaps you want to node something about it and need a little guidance.

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e2armory is the home for people who want to write and discuss firearms from a strictly technical perspective. We are a completely non-political usergroup, so please note that political discussion will result in expulsion. Everything from the musket to the modern day battleship cannon is on the bill of fare.

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Everything from cleaning solutions to firing solutions.

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Recoil operation is the name for a particular type of mechanical action in a self-loading firearm. It may be found in semi-automatic or automatic weapons. In general, weapons of this type utilize the conservation of momentum to harness the rearward impulse from firing - the recoil - to extract and eject the empty cartrdige and then chamber the next round to ready the gun to fire again.

This type of action is divided into long recoil and short recoil types.

The long recoil action is one where the barrel and bolt or slide of the firearm remain locked together as they recoil, all the way to the rear-most position of the mecahnism. They are then unlocked, and the barrel, under the influence of the recoil spring, returns forard first while the bolt remains locked to the rear and the extractor hooks to the cartridge. The barrel (and chamber) moving forward extracts the cartridge from the chamber. Once the barrel has returned forward, the bolt is unlocked and chambers the next available round.

In a short recoil action, the barrel and bolt travel together only a short distance; this travel is used to operate a the chamber locking/unlocking system rather than to fully extract the cartridge. At the end of the joined movement, they are unlocked and the barrel is stopped. The bolt (and cartridge in the chamber) continue to move rearward, extracting the empty cartridge and then chambering the next.

The long recoil system is mostly used in long guns, where there is more room for the system to operate behind the firing mechanism. John Moses Browning's famous Auto 5 semi automatic shotgun is a long recoil system. Very few handguns use long recoil, as the user's hand placement makes a long reciprocating action impractical. There have been some, however; the Frommer Stop, a Hungarian pistol from 1912, uses a long recoil design. Even earlier, a design known as the Roth-Theodorovic pistol was proposed for military contracts in Austria in the 1890s. Although it was never adopted or sold commercially, elements of its design went into the more successful Roth-Steyr 1907 handgun (a short recoil action).

Modern semi-automatic handguns generally use short recoil actions, with barrel and slide locking and unlocking by a variety of methods ranging from the 1911's tilting barrel freeing its locking lugs to the famous toggle lock of the Luger and the roller locked Heckler & Koch P9S or Korth PRS. In these guns, the barrel moves only a short distance, with the mechanism then unlocking the chamber and permitting the bolt and/or slide to continue rearward.

Recoil operation is generally more robust and requires less maintenance than a gas-operated system, and in addition the recoiling component can absorb some of the gun's recoil making it easier to fire for the user. However, to operate correctly, the gun generally must produce a recoil impulse within the design range of the firearm - meaning that in order to use ammunition that is more or less powerful than the design loads by enough of a difference, the action of the gun must be adjusted (springs can generally be changed).

Inertia operation is closely related to recoil operation. In the former, however, rather than the rearward motion of a component of the firearm under recoil operating the mechanism, the entire gun recoils and an isolated component remains still due to inertia; that component is used to unlock/operate the firearm.

An open bolt firearm in which the act of closing ('dropping') the bolt to close the chamber results in the loaded cartridge being immediately fired. By definition, the chamber on such firearms does not lock closed, and is sealed by inertia and spring pressure only, so it is generally only appropriate with less powerful cartridges.

Guns are designed with an open bolt for two general reasons. First, they are mechanically simpler; rather than having to have locking mechanisms for the chamber, along with associated mechanisms for safe unlocking such as delayed blowback, recoil operated or gas operated systems, the only moving mechanical parts need be the loading mechanism and the trigger mechanism. Generally, open bolt guns do not even have a separate moving firing pin; instead, a fixed pin is mounted on the bolt face, and when the bolt closes that pin strikes the primer. This simplicity can result in a cheaper, lighter and/or more reliable gun.

The second primary reason for using an open bolt is related to the first: if you are building an automatic weapon, the simpler mechanism is greatly to be desired for purposes of reliability, speed of operation and heat management. Generally, when an open bolt gun is cocked, the bolt is held back from the chamber by the sear, and the trigger releases the bolt to slam forward and fire the gun.

This comes with a significant accuracy penalty, which is why semi-automatics are only rarely open bolt (but some do exist). The penalty is the result of two factors: moving mass and lock time. The first should be fairly self-evident: when firing the gun results in a significant mass moving within it, the gun will likely deviate slightly from the point of aim due to the shifting mass before the round is actually fired. Second, this system means that there is a much more significant delay (known as lock time) between the moment the trigger sear is released and the time the cartridge fires, so the gun has more time to move off-target before the round leaves the barrel. This further complicates the first problem. In a fully automatic firearm, of course, accuracy at this level is less important than speed and reliability of operation - more rounds downrange can to some degree compensate for lack of pinpoint accuracy.

There are safety and environmental considerations as well. An open bolt gun presents, in most cases, more avenues for foreign matter to enter the mechanism when it is being carried in 'ready' state, which is why many such have dust covers which are either manually or automatically opened before firing. The safety issue arises when such a gun is carried uncocked, or uncharged. If a cartridge is in the chamber, movement of the gun can cause the bolt to move slightly back and forth due to inertia, and unless the bolt is immobilized by a safety mechanism (which not all have) it is possible for such disturbance of the gun to move the bolt far enough for its return trip to strike the primer of the ready round and fire it without the trigger being pulled. The M3 "Grease Gun", of WWII fame, had a dual-purpose dust cover for just this reason - when it was closed, it also locked the bolt in place to prevent this behavior.

Fully automatic closed bolt guns do of course exist. The AR-15 family and derivatives and the Heckler & Koch MP5 are just two examples of such. These weapons are much more useful in semi-automatic operation than open bolt guns.

A double stack magazine, also sometimes known as a 'staggered' magazine, is a box magazine for a firearm where the rounds are kept in two distinct columns (viewed from the front or back) in order to maximize the use of space. Since cartridges are (almost always) cylindrical, and magazines are oblong, a modest increase in width will allow the the rounds to be offset from side to side and allow significantly more ammunition to be carried in each magazine for the same vertical length of the magazine itself. This diagram won't show that due to the monospace font nature of it, but hopefully you'll get the idea:

/ O\
|O |
| O|
|O |
| O|
----

Smaller handguns typically do not use this system, as the increase in width of the grip is undesirable, and the concealability or small size of the gun is more important than having more rounds available.

These magazines come in two important sub types - the single feed and double feed versions. Single feed magazines have an opening at the top which is the diameter of a single round, usually in the center of the magazine. This makes the feed mechanism inside the gun simpler, as the designer will always know where the round will be presented. However, the fact that the rounds must 'shift' as they rise in the magazine from side-by-side to single-flie means that not only are the magazines more complex (they taper), and carry fewer rounds, but that complexity of motion means that jams and magazine-related malfunctions are more common.

Double feed magazines will present the rounds at the top on alternating sides, as they feed. This means the feed mechanism in the gun must take this into account, but nearly all modern firearms were designed with this in mind and it's not a problem. Double-feed magazines are generally more reliable, especially for high-speed operations in automatic weapons.

The primary reason to delay the blowback recoil operation of a firearm isn't to absorb recoil energy for the comfort of the user. At the moment of firing, the pressures inside the cartridge - and thus inside the chamber - are extreme, more than enough to split the cartridge wall and throw metal in whatever direction. The pressure rapidly drops as the bullet accelerates down the barrel (increasing the volume in which the majority of the gasses are trapped) and eventually the bullet leaves the muzzle, at which point the pressure drops dramatically.

The delayed blowback action is used when a firearm has a powerful enough cartridge that two things are true: one, the chamber must be kept mostly closed until the pressure has dropped far enough to permit the safe extraction of the empty cartridge and two, the recoil impulse is powerful enough that simple mass and spring tension isn't enough to perform this delay; some form of mechanical disadvantage must be used to further 'block' the chamber closed for long enough.

The Savage 1907 is the first in a line of semiautomatic pistols produced by the Savage Arms Company (named for its founder, Arthur William Savage) beginning in 1907. It is sometimes erroneously referred to as the 'Savage 1905' because the only year marked on the early guns is that year, indicating when the patents used in its design were awarded.

The 1907 is chambered in .32 ACP (also referred to as .32 Automatic or 7.65 Browning, the latter in Europe generally). This was an extremely popular civilian handgun cartridge in the early 20th century, and many so-called 'pocket pistols' were chambered for it. Although it would be mostly supplanted in duty and service pistols during World War One - which saw the widespread adoption of both the .45 ACP and the 9mm Parabellum as well as several other 9mm variants such as 9mm Kurz(.380), the Spanish 9mm Largo and the Italian 9mm Glisenti.

It holds a double stack box magazine capable of holding ten rounds of .32 ACP; for a semiauto of its time, that was an impressive capacity. The gun was advertised heavily for self defense, with advertisements showing women using it to fend off multiple attackers. The most common slogan was 'Ten Shots Quick!' Savage capitalized on this by getting testimonials from contemporary celebrities, such as William Cody and William Pinkerton, running alongside text asserting that the Savage would "fight like a fiend" to defend your wife should she need to use it.

The 1907 is a delayed blowback firearm. As such, it has a moving removable barrel instead of a fixed barrel mounted to the frame of the gun. it is nominally a delayed blowback - the barrel has a cam which fits into a channel in the top of the slide. The channel bends near the rearmost position; the theory is that when the gun fires, the slide and barrel are locked together by a single lug at the bottom of the barrel. As the slide moved backwards under the recoil impulse, the barrel would rotate due to that cam, unlocking the slide and permitting it to move backwards, extracting, ejecting and chambering a new cartridge. This, it was claimed, would delay the opening of the chamber long enough for the pressure to drop far enough to make this a safe operation. There are conflicting stories as to whether this mechanism actually works as intended, but everyone seems to agree that the 1907 is a quite reliable pistol even if it's just a fancy blowback design.

Several iterations of the gun would be produced between 1907 and 1929 when it was discontinued - the 1907 had many of its features modified and tweaked. The next major model was the model of 1915 in an attempt to regain market share being lost to bigger and more modern pistols. This included a version in .380/9mm Kurz to try to address the increasing popularity of larger cartridges. In 1920, the model of 1917 (yeah, the name is silly) was released, the last major revision of the gun.

Over a quarter of a million of this series of gun was produced (1907/1915/1917 in .32 and .380). In addition, there was a very small number (less than 300) produced in .45 ACP, known as the 'Government' model. Those pistols were produced for the pistol trials in which they competed against John Moses Browning's famous design which would become the 1911.

Sources:
Savage Arms: An Overview of the History,Development and Classification of the .32 and .380 Semi-automatic Pistols (Waldemar Goulet, Ph. D.)
Savage Automatic Pistols: Overview (Ian McCollum, Forgotten Weapons)
http://www.thetruthaboutguns.com/2013/10/jeremy-s/obscure-object-desire-savage-1907-45-pistol/ (The Truth About Guns, by 'Jeremy S')
Personal experience with the 1907