Sunday, September 19, 2010

Actions: Recoil Action: Muzzle Booster

A muzzle booster is a device that is sometimes used with some short recoil operated actions. It is used to add extra force to the recoil action. This helps boost reliability and increased rate of fire. The idea behind a muzzle booster is to tap some of the expanding propellant gases to add to the force acting on the recoiling parts.

A muzzle booster was first used on a Vickers machine gun of 1912. During World War II, it was used by the German MG-42 as well as the later MG-3 machine gun.

The basic example, as used on the Vickers machine gun consists of a flared cup attached to the end of the barrel. There is also an outer tube that surrounds the barrel as well. The outer tube is aligned very precisely so that it has an exit hole for the bullet to pass through after it emerges from the barrel. The outer tube also has other perforations to allow gases to escape.

When the bullet is first fired, it comes out of the barrel along with some burnt propellant gases. As it passes through the hole in the outer tube, it temporarily prevents gas from escaping out of the barrel. Therefore, the pressure in the outer tube rises and the gas moves backwards and pushes on the flared cup attached to the end of the barrel. This provides additional force to move the barrel backwards. The excess gas then escapes through some perforations on the outer tube. The animation below shows how this works (click on the image to view the animation).

Click on the image to view the animation.
Image licensed under Create Commons Attribution-Share Alike 2.5 Generic, 2.0 Generic and 1.0 Generic license
Created by user GraemeLeggett at

Muzzle boosters are also used when attaching silencers (or, to give the correct name, "suppressors") to short recoil operated pistols (i.e.) most modern semi-automatic pistols. Since the barrel and bolt are the recoiling parts of a short recoil operation, adding a silencer to the barrel increases the barrel weight. The extra weight may interfere with the recoil operation since more force is now needed to push the barrel backwards fully. Using a muzzle booster provides the extra force to counteract the extra weight added. This muzzle booster is sometimes referred to as a Nielsen device.

The same idea is also used for some blank firing adapters (BFA). During ceremonial occasions and military demonstrations, users are provided with blank cartridges that only contain the propellant, but not the bullets. Blank cartridges have lesser power than normal cartridges that contain bullets. Therefore, firing a blank cartridge produces much less recoil force, which means that it may prevent a recoil operated action from working properly. By screwing on a special muzzle booster (the blank firing adapter) to the end of the barrel, the extra force required to cycle the action is generated.

Saturday, September 18, 2010

Actions: Recoil Action: Inertia Recoil Operation

In the last two actions that we studied previously, the short recoil operated action and the long recoil operated action, the common feature to both is that the bolt and barrel are allowed to recoil, while the rest of the firearm remains immobile. Today we will study another type of recoil action called the Inertia Recoil Operated Action.

In this type of action, unlike the other two types, the bolt is held immobile initially and the rest of the firearm is allowed to recoil around it. This means that the recoiling parts form a greater mass than in the other two types of recoil actions that we studied previously. That implies that this action is suitable to be used by firearms with heavier loads, such as large bore shotguns. The original design for the inertia recoil action was done by Bruno Civolani in 1967. He tried to sell the design to several manufacturers, but was not successful until he approached an Italian firm named Benelli. Until then, Benelli was largely a moped and motorcycle manufacturer, but they were also exploring the business of firearms and they produced their first firearm using this action in 1969. The action became somewhat popular due to its fast rate of fire (about 5 rounds/sec). Since then, Benelli has manufactured a number of shotgun models that use this design and has also licensed the design to a few other manufacturers such as Franchi (Italy) and Stoeger (Turkey). Currently, the Benelli, Franchi and Stoeger brands are now all owned by Beretta S.A. of Italy.

Block diagram of the Inertia Recoil Operated Action. Public domain image.

In the above diagram, at step 1, bolt body A and barrel B are initially locked together and C is the frame of the weapon. The bolt body and barrel are joined together by a inertial spring. After it is fired (step 2), the bolt body A remains stationary while the firearm recoils into the shooter's body. As a result of this, the parts B and C move backwards and compress the inertial spring between A and B. Then in step 3, the inertial spring that was compressed in step 2 begins to expand and push bolt A backwards. At this point, the bolt A is unlocked and allowed to be pushed fully backwards as shown in step 4. The bolt has an extractor that pulls out the old spent cartridge case backwards and ejects it at this stage. The bolt moves backwards fully and compresses a recoil spring (not shown) at the back of the weapon and also re-cocks the weapon. Then in step 5, the recoil spring pushes the bolt forwards again and it picks up a new cartridge and the bolt is locked to the barrel and the cycle begins again.

The reader may be interested in an animated video of the above action, to get a better understanding of how it works.

The inertia operated system is known for its simplicity, since there are only 3 primary parts in the action: the bolt body, inertia spring and rotating bolt head. This also means the weapon is fairly light weight as well. Compared to gas operated actions, this action requires less cleaning since the fired gases do not enter the working mechanism and hence there is no carbon buildup. However, they generally have transmit a bit more recoil to the user than a gas operated system. As mentioned above, this is an extremely fast action compared to a long recoil operated action. It is also very versatile as it is capable of firing various types of cartridges of different firing powers, without re-adjusting the action at all.

As a demonstration of the speed of shooting an inertia action weapon, here's a video of a world class exhibition shooter, Tom Knapp, demonstrating how to shoot 10 clay targets with a single throw. Note that he mentions that his shotgun is a stock off-the-shelf model with only two extra accessories: a magazine extension to hold more rounds (the Benelli shotgun model he is using only holds 3 + 1 rounds without the extension) and his fiber-optic sight.

Benelli, Franchi and Stoeger are all known for their quality shotguns. The Benelli M3 shotgun in particular, is a favored weapon carried by SWAT teams.

Monday, September 13, 2010

Actions: Recoil Action: Long Recoil Operation

In the last few posts, we talked about a particular type of recoil action called the short recoil operated action. In this post, we will talk about another type of recoil action, the long recoil operated action.

This action is actually one of the older recoil operated actions in existence, being over 100 years old at least. It is commonly used in naval guns or artillery, but not as mucsh seen with small arms. When it comes to small arms usage, it is mostly used by some well known semi-automatic and automatic shotgun designs. The long recoil action shotgun was originally designed by the famous gun designer, John Browning in 1898 and patented in 1900. This action is very rarely used for pistols and there is only one (Frommer pistol) that used it for a pistol.

In a long recoil operation, the bolt and barrel are allowed to recoil, similar to that of a short recoil operation. At the point when the bullet is fired, the bolt and barrel are locked together. As the weapon fires, the bolt and barrel move back together due to the recoil. They continue to move together backwards until they reach the back of the receiver and recock the hammer. At this point, the bolt is held in the back of the receiver by a catch. The barrel is then pushed forward by a barrel spring and returns completely forward, during which time the spent cartridge case is ejected. When the barrel has reached its fully forward position, the bolt is then released from the back and pushed foward by another spring. As the bolt moves forward, it picks up a new cartridge from the magazine and pushes it into the barrel chamber.

As you can see in the illustration above, the first image shows the bolt and barrel right after the bullet has been fired. Note that the bolt and barrel each have their own individual recoil springs. The second image shows the bolt and the barrel moving backwards due to the recoil until they reach the end of their travel, whereupon the bolt hooks on the catch and is left held there. Meanwhile, the barrel recoil spring uncompresses and pushes the barrel forward, as shown in the third image. When the barrel reaches its forward position, it releases the catch that is holding the bolt back in the third image and the bolt is then pushed forward by the bolt recoil spring.

Compared to the blowback actions we've studied previously, this type of action can handle much heavier loads.

Compared to the short recoil operated action, there are some significant differences.
  1. When the weapon is fired, the bolt and barrel move back together initially, just like in a short recoil operated action. However, in the short recoil operation, the two move back together only a few mm. at most, before the barrel stops while the bolt continues to move backwards. In the long recoil operated action, the separation of the bolt and barrel happen much later. In fact, in a long recoil operation, the bolt and barrel move all the way backwards until they reach the end of their travel in the back of the receiver.
  2. Short recoil actions tend to have only one return spring. Long recoil actions have separate return springs for the barrel and the bolt.
Since the bolt and barrel are much heavier than the bullet and since they move backwards together a long way compared to a short recoil operation, the cycle time of shooting and reloading is much slower compared to a short recoil action or a blowback action. On the other hand, the longer cycle time leads to slower, smoother operation.

The long recoil operated action was used in the John Browning designed Auto-5 shotgun, which was the first semi-automatic shotgun. This shotgun was originally designed in 1898. The Browning Auto-5 model was designed with the intention of making it suitable for mass production and the design was licensed out to various manufacturers, such as FN, Remington, Savage arms, Franchi etc. and remained in production until 1999. This makes this model one of the most successful shotguns in history.

Remington Model 11 shotgun, which uses the Browning designed long recoil action.
Public domain image.

When John Browning had originally designed the Auto-5, he intended to sell the design to the Winchester arms company, since he had already sold several designs to them previously. However, Winchester was not prepared to pay the amount of royalty that Browning demanded, so he went to Remington next. Tragically, the Remington deal fell through, as the president of the Remington company died of a heart attack during the negotiation period. Hence, John Browning went to Europe and licensed the design to Belgium's Fabrique Nationale (FN). Later on, Remington licensed the design again and used it with their Model 8, Model 11 and Sportsman model shotguns. The design was also later licensed to Savage Arms of the US, Franchi and Breda of Italy and Tula State Arsenal (TOZ) of Russia. The French designed Chauchat light machine gun of World War I also used a version of the Browning long recoil action mechanism. While the Chauchat was mostly a failure, the shotguns remained in use for a very long time indeed.

Thursday, September 9, 2010

Actions: Recoil Action: Short Recoil Operation - II

In our last post, we studied the basics of a Short Recoil Operated Action, as well as some different weapons that use it. In this post, we will study one more variant of the short recoil operated action, the toggle lock recoil action. This variant of the short recoil operated action was originally used by the world's first machine gun, the Maxim. It was later used in some small arms, notably the Hugo Borchardt designed C93 pistol and later on by the Luger pistol.

The C93 pistol was one of the first successful commercial semi-automatic pistols. Hugo Borchardt was a German born inventor, who later emigrated to the United States, became an American citizen and worked for several well known American firearms manufacturers, including Colt, Winchester and Remington. He later emigrated back to Europe and was employed by the Ludwig Loewe company when he invented his pistol in 1893.

The copyright holder of this file allows anyone to use this image for any purpose, provided the copyright holder is properly attributed.

The pistol was fairly successful, despite the fact that it was mechanically complicated, expensive to produce and hard to handle. Note the almost vertical grip and the big, ugly, semi-circular mainspring hanging out of the back of the weapon. These two features make it harder to aim the pistol quickly and affect the overall balance.

Hugo Borchardt was an easily offended man by nature and he believed that his design was perfect and couldn't be improved. As a result, another employee of the Loewe company, Georg Luger, was tasked with improving the design, which he succeeded in doing in 1896. The Luger design started with the basic Borchardt designed mechanism, but had an angled grip that made it easier to point, added a new grip safety, removed the C93's mainspring and replaced with a leaf spring, improved the balance, redesigned the cartridges used etc. The new pistol was originally adopted by the Swiss army in 1900 and was later adopted by the Germans in 1908. Since the Germans adopted it in 1908, they named it as Pistole-08 or P-08 for short. The words for "zero eight" in German are "null acht", hence this pistol was also known in Germany as the "null acht". The pistols originally used 7.65x22 mm. cartridges, but Luger invented a different sized cartridge for the German army and redesigned the pistol to use this new cartridge: the famous 9x19 mm. parabellum cartridge, also popularly known as the "9 mm. Luger" cartridge. The 9x19 mm. cartridge is still with us today and is the most common military handgun cartridge in use currently.

The Luger was used heavily by the Germans in World War I, produced by both Erfurt Arsenal and DWM (the Loewe company had been renamed to Deutsche Waffen und Munitions Fabriken (DWM) by this time). After the war, the allies restricted production of most weapons for a while, until a German company called Simson was allowed to manufacture Lugers after 1925. Ironically, the Nazis later forced the Simson company to close before World War II, because the owners happened to be Jewish! The Mauser company continued to manufacture Lugers until the end of World War II. Lugers were later replaced in military service by more modern designs, but there are still some private manufacturers making Lugers to this day.

The most distinctive feature of a Luger (and its C93 predecessor) is its toggle lock mechanism. It works similar to the principle of a human knee. Note that a human leg can withstand a lot more pressure when the knee is straight and locked, but once the knee is bent, the leg suddenly becomes much easier to bend after that. The toggle lock mechanism works very similarly. When the weapon is initially fired, the toggle lock mechanism is straight. The recoil causes the barrel and toggle lock initially move back together on rails. Due to the toggle being straight, it doesn't move backwards easily initally. After a little bit of movement, the toggle part begins to ride over a pair of cams that bend the toggle at the joint. Once the toggle lock is no longer straight, it bends much more freely, allowing the bolt to accelerate backwards and recock the weapon. An extractor at the end of the toggle pulls out the old cartridge and ejects it. A recoil spring then pushes the bolt forwards again, whereupon it strips a new round from the magazine and pushes it in the chamber, ready to fire. The movies below show how this works:

This design had a few disadvantages though. For one, it was a fairly complicated mechanism and required precisely fitted parts to work properly. The tolerances required for this design are very tight and parts often had to be hand-fitted for it to work right. These tolerances contributed greatly to the accuracy of the weapon, but took away the reliability. In fact, small amounts of dirt on the exposed firing mechanism parts, especially on the left hand side, could cause the weapon to jam. The Luger was expensive to produce, even for its time, and it was competing in the civilian market with the Colt M1911, which was cheaper to produce and more reliable in operation.

This is why there are no other modern weapons that use this particular variant of the short recoil action mechanism.

Tuesday, September 7, 2010

Actions: Recoil Action: Short Recoil Operation

In our previous post, we studied the basics of recoil operated actions. In this particular post, we will study one of the types of recoil operated actions, i.e. the short recoil operation.

This action type has a long history, being the action that was used in the first machine gun ever invented, i.e. the Maxim machine gun. It is also the action of choice for most semi-automatic and automatic pixels that use 9 mm. Luger (9x19 mm.) cartridges or bigger. Smaller and lower powered cartridges are used with blowback actions that we studied last month, but once the cartridges get beyond a certain size and power, a blowback action cannot be used any more and most modern pistols therefore use a short recoil operation.

As we noted in our previous post, one of the key differences between a blowback action and a recoil operated action is that a recoil operated action has the bolt (i.e. the metal block that holds the cartridge in the chamber before firing) is locked at the point of firing, whereas a blowback action merely has it held under spring pressure. This enables the recoil operated action to fire heavier cartridges. Also, as we noted previously, in a recoil operated action, some parts of the weapon are allowed to recoil backwards, whereas other parts are held stationary relative to the recoiling parts. In a short recoil action, the parts that are allowed to recoil are the barrel and the bolt.

At the point when the bullet is fired, the barrel and the bolt are locked together and move backwards due to recoil. After a short distance of travel (a few mm. in the case of pistols and a few caliber lengths in the case of machine guns), the bolt and the barrel disengage from each other. How this is achieved is what differentiates various short recoil operation systems. In some cases, the barrel is slowed down, in other cases, the bolt is accelerated. In either case, the barrel stops moving, whereas the bolt continues to move backwards and compresses a recoil spring and also extracts the old cartridge case at the same time, which is ejected via a side port. The recoil spring then pushes the bolt forwards and as the bolt moves forwards, it picks up the new cartridge from the magazine and pushes it and the barrel forwards. When the barrel reaches its forward position again, the bolt and barrel lock again and the weapon is ready to be fired.

As was mentioned above, in some systems, the barrel is slowed down to separate the barrel from the bolt. Weapons that use this include the classic Colt M1911 pistol designed by John Browning, all Glock pistols, Smith & Wesson, FN Browning, SIG Sauer etc.

In the case of the Browning designed Colt M1911, the bolt, barrel and slide all move backwards together initially, but then a barrel link tilts the barrel downwards. As the barrel tilts, the barrel locking lugs unlock themselves from the slide recesses. The bolt and slide continue to move backwards and an extraction claw on the slide also pulls the spent cartridge out of the chamber.

An ejector strikes the back of the spent cartridge and pushes it out of the extraction port, as the slide and bolt continue to move backwards. When the slide and bolt reach the back of the pistol, the recoil spring pushes them forward and during the forward motion, the slide locks back into the barrel and the three pieces then move back together. The two animated movies below show how this works.

In the case of Glock, SIG Sauer and most recently designed automatic pistols, this mechanism is even more simplified. There is a downward inclined ramp attached to the back of the barrel. As the barrel and slide move backwards, the ramp contacts a fixed stud on the pistol's frame. This causes the back of the barrel to move down (and the front to tilt up), which moves it out of engagement with the slide. The rest of the action is similar to what was described earlier. The animation below shows exactly how this works:

In the above three animations, the one thing is common is that the bolt and barrel initially move backward together and are then separated by slowing down the barrel while leaving the bolt and slide free to continue moving backwards.

Another way to achieve this separation is by accelerating the bolt after they both move back together. This mechanism is most commonly used today in the 7.62 mm. and 12.7 mm. M2 machine guns invented by (surprise, surprise) John Browning! The 12.7 mm. M2 is still used in the famous Abrams tank, as well as most other western tanks, which shows the reliability and timelessness of this design. In the M2 machine gun, the barrel and bolt initially recoil together about 10 mm. backwards and then a rotating cam in the receiver disconnects the bolt from the barrel. The barrel has an extension in the back and bottom of the barrel. After the bolt and barrel are disconnected, this extension then strikes a short, curved lever. The lever is pivoted so that it has a mechanical advantage and therefore it pushes and accelerates the bolt rearward. A fixed stud stops the barrel extension, which also halts the barrel, while the bolt continues rearward. As before, we have an animation to show how it works:

We will continue studying short recoil operations tomorrow, where we will deal with some more historical short-recoil operated weapons such as toggle-bolt short recoil actions. It'll be interesting reading!

Sunday, September 5, 2010

Actions: Recoil Action

In the previous month, we made a detailed study about the Blowback Action and various ways to utilize this principle. In the next series of posts, we will study an action called the Recoil Operated Action.

The basic principle behind a recoil operated action is Newton's third law of motion. In simple terms, it says "For every action, there is an equal and opposite reaction." Thus, when the bullet leaves the barrel, there is a recoil by the firearm in the opposite direction. Due to the law of conservation of momentum:
mass of bullet * velocity of bullet = mass of firearm recoiling parts * velocity of firearm recoiling parts.

The velocity of the bullet is high, but since the mass of bullet is much smaller than the mass of the firearm, therefore the velocity of the recoiling firearm is correspondingly smaller than the velocity of the bullet.

In recoil operated firearms, the entire firearm doesn't recoil when the bullet is fired. Instead, only a portion of the firearm is allowed to recoil, while the rest of the firearm remains motionless relative to the recoiling parts. The recoiling parts and non-recoiling parts are connected together by a spring, which is used to return the recoiling part back to its original position. Unlike blowback operated firearms, the bolt is held locked at the time of firing. This means heavier cartridges can be used with recoil operated firearms.

Similar to the blowback action, it is desirable for the bolt to start moving only after the bullet has left the barrel and the gas pressure in the chamber has fallen to safe levels. This is because: (a) a tight gas seal must be maintained until the bullet has left the barrel for greater range. If the bolt moves back early, it will reduce the gas pressure in the chamber and barrel and reduce the force propelling the bullet out of the barrel (b) It is not good for high pressure gas to blow through the internal mechanisms of the gun and rearrange everything on the way out.

As we've seen previously, in a blowback action, this effect is achieved by using the inertia of the bolt and spring pressure to hold the bolt in place at the moment the weapon is fired. The bolt is not locked in any form of blowback action. In all recoil actions, this effect is achieved by locking the bolt to the chamber and delaying the unlocking until after the bullet has left the barrel.

There are three major types of recoil operated firearms:
  1. Long Recoil Operation: This is mostly used in automatic shotguns.
  2. Short Recoil Operation: This action dominates in automatic machine guns and semiautomatic pistols.
  3. Inertia Operation: This is a newer action that is used in some bigger shotguns.
In the next few posts, we will study these various actions in some detail.