Design of Automatic Weapons_Puneeth_abhik

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ME-401 Machine Design G ro u p P ro j e c t

DESIGN OF AUTOMATIC FIREARMS A QUALITATIVE AND QUANTITATIVE DESCRIPTION OF BLOW-BACK MACHINE GUNS

BY: Abhik Patel N.C. Puneeth

Department of Mechanical Engineering • IIT Gandhinagar• email: [email protected] • Ph: +91 9173437836

Design of Automatic Firearms §1. Evolution of Weapons Man has a long standing legacy of using weapons - he has been using them ever since the Old Stone age, albeit the purpose has changed a lot since. Originally, weapons were used to hunt for food and collect wives. But gradually weapons began to be used for for self defense, protection from wild animals, and eventually for aggressive purposes, and to assert power.

Figure Shows the some Prehistoric Weapons Originally, the club was the most common weapon wielded by man. It remained his favorite weapon for long, with many modifications made by increasing the mass, adding spikes (in scepters) and use of metal.

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Design of Automatic Firearms

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Figure shows (From Left to Right) : A Club, A Morning Star, A Flail (Courtesy: Diablo Wiki Website) The clubs damage was mainly by crushing, via blows of high impulse. The other class of weapons, that damaged mainly by cutting or slashing included swords, knives, lances, scimitars etc. Over the times, man learned to make swords that were more effective in battle by offering higher strength, and greater dexterity due to their lightness.

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! Figure Shows some popular models of swords (Left to right) Short Sword, Sabre, Scimitar, Falchion, Long Sword (Courtesy: DiabloWiki Website)

With repeated experimenting, the primitive broad sword was modified to add to its slicing power. The crescent shaped scimitar was contribution of the Far East, which was a formidable weapon to face in battle. In melee weapons, another class of armor piercing weaponry was the pole-arms, which involved sharp blades attached to a long pole. By this time, significant advancements in terms of war mobility and movement were made, and cavalry (horses) were being used for basic mounted units. The first big breakthrough came with the use of projectile weapons. The credit for inventing the sling goes to the Phoenicians. The slingers were of he most feared adversaries in battle then. By attaching sharp stones to clubs, the stones were projected at much higher speeds than could be achieved by human arm. The invention of the bow and arrow revolutionized projectile weapon technology. A variety of bows with a range of power, accuracy and reliability were developed.

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! Figure shows some famous Bow models (L to R) Short Bow, Composite Bow, Long Battle bow, War Bow (Courtesy: DiabloWiki Website)



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Now we quickly accelerate to the invention of Gunpowder in approximately 9th Century A.D by Chinese alchemists, who were trying to produce the Elixir of Life. Gunpowder was created accidentally by mixing Saltpeter with a Nitrate salt. The Chinese were known to use incendiary arrows, which were tipped with gunpowder, causing devastating flames upon impact. Human warfare now reached Gunpowder age. By 13th century, actual fire arms were being used in form of heavy siege cannons. The initial fire-arms were difficult to load up and fire, and elaborate preparation was required to fire a round. This lead to further experimentation in ignition. and loading mechanisms. An interest is also noted in multifiring systems, that had multiple barrels to fire a volley of shots in one go.

3 - Barrel Matchlock. If the people from these historic era were to face today’s modern war equipment, they’d surely be in for a shock. A small army equipped with today’s automatic precision rifles can consume what would have been regarded as a huge army wielding primitive weaponry - not to mention usage of artillery, tanks, bombs, air warfare, and of course, nuclear power. §2. Weapon Design and Our Interest: The description of the evolution of weaponry done so far brings to light an important fact - man has continuously invested efforts to develop more accurate, powerful and complex armament. Weapons are a direct psychological advantage, an instrument to security and a means to overcome physical limitations of power and control. That is why the study of weaponry has always been appealing to man. The story of the development of arms outlined the gradual evolution in the understanding of design principles based on repeated experimentation and understanding of science. A more complete reference dealing with why we need weapons is given in “Evolution of Weaponry” by Lt. Col. Dave Grossman[1]. While the above argument is at a psychological level, at a social level, gun makers and gun designers were given high credibility. That attracted many who were skilled in mechanics to work in this area, boosting firearm study. The following excerpts from “The Machine Gun” by Lt. Col. George. M. Chinn illustrate this:[2]

Figure shows two passages from [2], which illustrate how firearm making was a profession much seeked-for Till date, the most powerful weapons are those that are able to deliver a high rate of fire with higher power. Its all in the numbers - you fire more bullets, you have a greater chance to hit. Your gun is more powerful, your hits have a greater chance to be fatal. That has precisely been the reason for general interest in automatic firearms. Automatic firearms are able to deliver large rate of firing along with accuracy and power. But the development, evolution and design of automatic weapons has by no means been a simple one time affair. Automatic weapons present a wide variety of challenges, I I T G a n d h i n a g a r!


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in design and manufacture, and have been studied extensively over the last few centuries. The design of guns is more or less a legacy now passed over generations by gunsmiths across the world. Enormous data has been collected, analyzed and optimal designs have been developed based on the empirical observations from the experiments. Mathematical models of the operation of automatic weapons have also been produced, sometimes, out of purely academic interest in order to better visualize and understand the functioning and limitations of these weapons. In this paper, we take a brief tour of such attempts on modeling the operation of automatic weapons, from a purely design perspective. There are several models which explain the existing devices, but only we present only one in full detail, along with an Excel Based tool with the mathematics coded to aid in visualizing numbers associated with the design. Glimpses of the other models are presented for completeness. Today’s automatic firearms are highly robust, precise, reliable and accurate devices. But the automatic firing is largely based on one common principle - using the energy from the shot to re-cock the gun. Obviously, it is not as simple as it sounds, and it has several levels of complication due to various factors involved. Lets take a look at the design of automatic weapons to understand what these concerns are. §3. Design of Automatic Weapons: Automatic Weapon is a weapon which is designed so that a round is loaded mechanically after a previous round has been fired. The firing cycle consists of a step where the used cartridge is ejected and a new cartridge is fed into the barrel, before the next firing sequence is initiated. Invariably, the energy for the automatic action is derived from the explosion of the cartridge, through the use of different possible specialized arrangements designed optimally to produce automatic action. There are several types of automatic weapons, based on the round used, type of automation, firing rate, number of barrels, etc. Some of the famous types, are Automatic Pistols (e.g. Steyr TMP), Automatic Shotguns (e.g Daewoo USAS-12), Automatic Rifles (e.g. Browning Automatic Rifle), Submachine guns (Hand held variety of machine guns, e.g. MP5 Carbine) Machine Guns (e.g. M2 Machine Gun) and Gatling Guns (e.g. M61 Vulcan Gatling Gun) .

! Automatic Weapons (Left to Right) : Steyr TMP, Daewoo USAS-12, MP5 Carbine The design of Automatic Arms, as mentioned, mainly involves a mechanism to eject a cartridge and load another between firing intervals. Such a mechanism is usually powered by the energy of the round itself, though in many of today’s aircraft mounted Gatling guns, the barrel rotation is powered by an electric motor, which derives its high power requirements from the aircraft’s hydraulic systems. Invariably, in all hand held, or fixed low and heavy caliber machine guns, the energy of the implosion that drives the projectile out of the muzzle is harnessed for achieving automatic action. There are several mechanisms available today for this purpose. These designs are based essentially on few primary ideas, and are usually more evolved versions with added mechanisms to optimize firing rate, life etc. In this section, a brief description of the design aspects of each of these models will be given.



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§4. Classification of Automatic Firing Mechanisms: The classification of automatic firing weapons is based on how a part of the energy from the implosion of the gunpowder is used for automatic action. The are four major kinds of automation: 1.

Blowback based systems

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Recoil Operated Systems

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Gas Operated Systems

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Multiple Barrel Systems

Most of the designs are based on one or a combination of these design ideas. Even within these kinds, there are many subclasses, each adding a distinctive feature to the working of the gun. Some of the classification schemes can be visualized by the following charts (Courtesy: [3])

Figure Shows the classification of Automatic Weapon Mechanisms In this project, only the Blowback mechanism is considered for design analysis. The other mechanisms are currently out of scope as far as the project is concerned.



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§5. Blowback Mechanisms: For the design of any gun the essential criterion is that the cartridge case must be sufficiently supported to withstand the tremendous thrust applied by hot gases emanating from the implosion of gunpowder. If the case ruptures (called case separation), the gases in the barrel remain unrestrained at a high pressure and can blow the whole gun up, seriously injuring the user or sometimes even killing him. Some of the images below depict blown up guns -

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Blown up Bolt of a Broken 1902 7 x 57 smokeless action gun (Courtesy:[4])

In most guns, a locking lug is present that holds the cartridge in place against the high pressures developed during the firing. For design of automatic weapons, locking poses a problem, because the cartridge has to be ejected between firings automatically. So the locking mechanism for an automatic weapon must be designed in such a way that it locks the cartridge case as long as the pressures int he chamber are high and releases it immediately for ejection. In this section similar design challenges related to the design of blowback operated guns are presented.



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The figure shows a schematic of a Blowback Mechanism When a cartridge is fired, hot gases emanating from the implosion rapidly increase the pressure in the barrel and the case is thrust backward towards the bolt, which is referred to as “Blowback” which is the basis for design in this mechanism. The blowback action occurs when the bolt is left free to move. If it is locked with the help of locking lugs, all the gas is forced forward, leading to no blowback. The bolt’s controlled motion is an important requirement for automatic weapons designed based on blowback mechanism. When the bolt retracts due to blowback, the extractor pulls the spent casing into the chamber. A driving spring then forces the bolt forward, during which another cartridge is stripped of its magazine and pushed into the chamber. After firing, the ejector kicks the case out of the gun. This action (known as “plain blowback”) is illustrated and described in the following paragraphs.

Initial State: In the above figure, blue portion is the bolt. The bolt handle A is used to pull the bolt backwards initially against spring pressure from recoil spring B. This is the initial cocking of the gun. It retracts the whole bolt backwards. C is a cavity in the bolt, where hammer E (in pink) can move. D (in red) is the firing pin. When the gun is initially loaded, a magazine containing cartridges is first inserted into the weapon and the hammer E is cocked using handle A. At the same time, due to the pressure of magazine spring F, a new cartridge is pushed upwards into the receiver. When the bolt handle A is released, the front face of the bolt (the blue part) pushes the cartridge forward so it is pushed into the chamber in front of barrel H. The hammer E still remains in its near horizontal position because it is held in place by the trigger bar (which is orange) connected to trigger G.



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When the trigger is pulled: The trigger bar releases the cocked hammer E, which rotates about its axis inside cavity C and strikes the firing pin D. The other end of the firing pin strikes the base of the cartridge, which detonates its primer and then ignites the main gunpowder in the cartridge. The expanding gases drive the bullet through the barrel H and also push back on the empty cartridge case left behind in the chamber, which in turn pushes back on the bolt. This part is unique to the blowback operation guns. It is shown in the following figure.

As a result of the blowback, the hammer E is now rotated back down until the orange trigger bar holds it down. Meanwhile, the opening on top of the receiver is now open and the old cartridge case is pushed out through here by the action of magazine spring F. When the bolt has moved backwards to its utmost, the recoil spring B then expands and pushes it forward into place. While moving forward, the bolt picks up the new cartridge and pushes it into the chamber and the gun is now ready to fire again. This is the basic mechanism of operation of blowback based automatic guns which are quite common today. In the following section the concerns with the design are explained.



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§6. Design aspects of the Blowback Mechanism: The most important consideration for the design, as mentioned earlier is the study of the behavior of the case under the high pressure conditions created by the action of the hot gases. The pressures reached are high indeed, reaching up to 50,000 psi within a short period of firing. The pressures reached in the muzzle differ for each kind of ammunition. It can be measured using some simple pressure transducers attache to the gun muzzle. Here is what a typical transducer output looks like:

The test was performed with a 70 gm. TNT based ammunition. We can see that the pressure is initially low but rapidly raises up to 40,000 psi, and then gradually falls. Here, rapid and gradual are just relative, the whole thing happens in just about 1 - 1.5 milliseconds. For a high powered machine gun, a representative ammunition would be a 20mm round weighing about 0.29 pounds. Since data and calculations for such a round were available, we will be analyzing that here.



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From the above chart it is evident that the pressure variation has 3 phases, first of rapid pressure rise, second of pressure loss as the bullet travels in the muzzle, and final phase where after the projectile leaves the muzzle the pressure drops back to normal rapidly, due to the dispersal of the gas.

Schematic showing the building of pressure in the chamber (Courtesy:[3]) In phase I when the pressure rises, the radial forces on the case cause it to expand against the walls of the chamber, creating a kind of seal, which prevents the gas from escaping backwards. The radial forces acting on the case are very high, and so the case (which is usually made of semi hardened bronze material) easily expands to fill up the chamber, sealing it up for the gases. But in case the pressure is not enough to for a strong seal, i.e. if a smaller (wrong) calibre round is used with a chamber of much higher bore, rearward escape of high pressure gases can cause the gun to blow up (see figure below). Hence, great care must be taken to use designated ammunition.

Gun Blowing up due to wrong ammunition The case not only takes up radial load, but also takes up axial tensile load. This load arises from the pressure exerted by the gases near the shoulders of the casing. The figure below illustrates this. We can see that the total pressure can be split up in two parts, pressure producing motion of the case, and pressure simply producing tension in the case (equal and opposite pressures). However, it is worth noting that if the radial pressures are so high, that the case is tightly set against the walls of the chamber, then the case will be unable to move due to the frictional forces, and the component of pressure otherwise producing motion would also simply contribute to the tension in the case. Experiments have shown that is catastrophic, for the pressures reach really high values and if the case is not allowed to move, it ruptures (case separation) under tension, causing the gun to blow up.



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Figure shows (schematic) of pressure affecting cartridge case (Courtesy: [3]) In the second phase when the pressure becomes very high (~45,000 psi) as mentioned, if the case is not allowed to move freely, the gun may get jammed or can even blow up. Hence, suitable lubricants have to be employed to form a sufficiently thick film around the case, so that there is no metal to metal contact even at high pressures. Since the pressures are high, think film lubricants are not effective, and the deformation in the casing can itself cause metal to metal contact (due to uneven expansion). Several brands of oil are available for use as lubricants. One such lubricant that is popularly used is the Miltec -1. The adjoining figure (Courtesy [3]) illustrates why using a thicker oil is necessary to operate the blowback mechanism. Another advantage of using thicker lubricant is that whenever the gun is fired, due to the ignition of the gunpowder, a flame is produced that travels outwards from the case, and lighter oil is highly susceptible to drying up, when compared to heavier oil. Heavier oil films limit the escape of the flame better persisting the pressure and making the weapon more effective. The above discussion shows that lubrication is a vital aspect for design of blowback mechanism guns. There are several other concerns with usage of lubricants like oils such as leakage contamination, burning, etc. which are not considered in this simplified design analysis. However, these problems can be critical and hence demand equal, if not more attention of the designer. Last but not the least, the design of the case itself is an extremely important aspect.Though it is not a part of the gun design, the gun designer must definitely keep in mind the available ammunition while designing his gun, so that some round is suitable for use with the gun. The strength of the case (must be high) , the material used in the case (it must be highly elastic), and surface finish (must be well polished to minimize friction) have to be accounted for while designing the gun to be used with a certain round. I I T G a n d h i n a g a r!


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§7. Design Plain Blowback Systems: The cycle of operation of plain blowback systems was described in §5. Here we consider the design aspects of these systems. The limitations for the design of a blowback based gun come up in terms of restricting the movement of the bolt. During the high pressure phase, the cartridge case suffers heavy tensile stresses, and the bolt has to restrict the elongation so that the case does not rupture. Empirically for 20 mm rounds, this amounts to an elongation of less than 0.015 inches for the first 0.0015 seconds[3] Approximating constant velocity, this must not exceed 10 inches/second. This serves as a guideline for design of the bolt. However, there is also another limitation. If the case expands and moves so rapidly, that it pushes the bolt back, and moves out of the chamber, the internal pressure will cause the case to swell and burst. Thus, the case must somehow be restricted to stay inside the chamber till the pressures are low enough for it to exist without the chamber walls backing it up. Empirical results broadly decree that the movement of the case must not exceed 0.250 inch in the first 0.010 seconds. Approximately assuming constant velocity, it should not exceed around 25 inches/second.[3] Design for blowback operated automatic guns revolves around these two constraints. Thus, even if lubrication is employed, to prevent case separation following the first rule, the case should not be moving so fast that the second rule is violated. The following plot shows a typical bolt velocity vs time plot before the forward stroke of the bolt due to the driving spring.

Figure shows the bolt velocity vs Time graph. The average velocities are much lower than the limits here. The design guidelines seem ok, but if such a gun is built it will be far from useful. Out of several reasons, two of the most pertinent reasons are discussed here: Firstly, the weapon will have mediocre firing rate. A quick calculation will reveal this. If a 20 mm cartridge has to be accommodated, the gun must open about 12 inches to permit feeding. In one open close cycle the bolt must travel about 2 feet (2 x 12 inches). The limiting average speed in 25 in/second. which means at best there can be one reloading in 1 secI I T G a n d h i n a g a r!


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ond, which, with other reductions will about to a speed of around 50 rounds per minute. That is a severe limitation on the capacity of such a gun. Secondly, it is worth investigating what the bolt must be like to have such a constrained motion. The bolt is retracted due to its collision with the case. Lets plug in some numbers to see what characteristics the bolt must have. For a 20 mm round, as seen earlier the maximum pressure in the barrel reaches 45,000 psi. At this pressure, the driving force on the case (with appropriate dimensions considered) will be around 22,000 pounds, reduced only by the viscous shear applied by the lubricant, which is hardly significant. The driving force still remains in excess of about a 20,000 pounds. When it collides with the bolt, the spring is totally loose, so it does not contribute significantly in resisting the motion of the bolt. The speed of the bolt is totally dictated by the physics of momentum transfer, which in turn depends on how massive the bolt is. Through momentum conservation we know that if the bolt is just as massive as the projectile, both will blast off with almost the same speed. If that happens, despite the springs resistance, the bolt will impinge into the back of the gun and blow it up. So the bolt has to be made much heavier when compared to the projectile. Calculations reveal that this could be as high as 500 pounds for a 20 mm round. Just the bolt weighing 500 pounds is impractical. One because it would be difficult to maneuver the gun. If the muzzle is raised slightly above horizontal the spring would retract causing the embarrassing condition of failure to fire. Also, for a higher mass, the amount of energy absorbed in collision will be lower, and hence there will be lower energy available for the operation of the mechanism. With a 500 pound bolt, the mechanism will probably not operate at all. Thus, the plain blowback based guns are not suitable for practical design. They however form the proof of concept for a whole regime of blowback based guns. By incorporating certain modifications to the design, these problems can be addressed. These modifications will be mentioned briefly in the next section. To summarize, the following calculations are the quintessence of designing plain blowback guns: 1.

Determination of the bolt weight to limit recoil velocity to a safe value

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Determination of the data necessary for the driving spring, based on loading, extension, and fatigue considerations.

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Computing the rate of fire

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Computing the power absorbed by the bolt. This tells us how much energy is available for the blowback mechanism to operate so that other parts can be designed accordingly.

The mathematics for this part is to be developed on a separate Excel Spreadsheet™ which shows these calculations. In the following sections some modifications made to make the blowback system work better are explained.

§8. Modifications on the Plain Blowback Gun

1. Advanced Primer Ignition: We saw in the plain blowback case that the ignition occurs when the bolt is at rest. When the charge ignites, the bolt takes the full burst and is blown back with maximum possible impact. The only resistance to it is inertia (or mass) of the bolt. But if the ignition occurs before the bolt reaches the position of rest, i.e. when it is in its forward motion due to the driving spring, some of the energy will be consumed in stopping the bolt, and thus a lower mass for the bolt can be employed in design. This is referred to as “Advanced Primer Ignition”. It not only reduces the mass of the bolt considerably, but allows the bolt to move at higher speeds (because of the advanced ignition, it has to travel a longer distance in a shorter time, so the average bolt speed limit is increased) which leads to higher firing rate. Thus, advanced primer ignition has two major benefits, significantly lower bolt weight and faster firing rate. I I T G a n d h i n a g a r!


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The advanced priming is achieved by an alternate firing pin, which works off phase with the bolt action. It is depicted in the figure above. The ignition occurs before the bolt reaches the breech casing. Calculations reveal that is advanced priming of 0.0001 seconds is applied, the maximum bolt speed is increased to about 30 inches/second (2.5 ft/second). Though it is an improvement, it is not high enough yet. The weight reduction factor of the bolt is found to lie between 2 - 2.5 i.e. the bolt can now weigh about 200 pounds. The calculations of this part too are meant to be performed on an Excel Spreadsheet for this project. 2. Delayed Blowback: In the previous section advanced primer ignition was employed to reduce the bolt mass. It is by now evident that the major problem with designing blowback systems is the control of excess motion of the bolt, which if not regulated will blow the gun up. In delayed blowback arrangements, a locking system is used to hold the bolt in place for a certain fixed duration, until the peak pressures have passed and the bolt is unlocked at a safe operating pressure, causing controlled motion. The unlocking time can be controlled by making precise adjustments to the mechanism. Depending on the locking time, the amount of energy transmitted to the bolt on release will vary. This is adjusted according to the needs of the gun. The locking systems employed are numerous - they can be based on recoil action, gas action, booster actuation, mechanical devices etc.



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In delayed blowback systems, the bolt weight can be brought down considerably, as desired by modifying the unlocking point. With suitable adjustments, the bolt weight can be brought down to as low a value as 10 pounds, which is a very good figure. Manipulation of the locking time also improves the firing rate, as the bolt can travel much faster in the lower pressure regime, without the danger of the case rupturing. However, if the unlocking is done at moderately higher pressures, there is still a danger of case rupture. If the unlocking is done at too low pressures, the amount of energy to make the mechanism work will be too low. So, and optimal value has to be determined for this purpose. 3. Retarded Blowback: In this kind of arrangement, the bolt motion is controlled with a separately placed retarding mechanism, that “brakes” the motion of the bolt to a safe zone. This provides the controlled bolt motion required for the working of the blowback system. On such retarding system is illustrated in the following figure:

! Figure shows the schematic of one possible retarding mechanism The mechanism uses a “toggle” mechanism, to retard the bolt motion. The “toggling” action drastically reduces the speed of the bolt and also changes the direction of motion. However, the design difficulty now shifts to the design of the linkage that will bear the load. The real advantage with the retarded blowback system is that the resistance to motion is not uniform. It is adjusted in such a way that at the peak of the pressure cycle, the mechanism remains at a toggle position minimizing the bolt motion and maximizing safety and control. Elaborate calculations are available to analyze the exact retardation pattern to aid in the designing of retarded systems. These calculations are intended to be made on an Excel Spreadsheet in future. §9. Other Types and Scope Blow back operated automatic guns are by no means the only type of automatic guns. As mentioned other popular types include Gas operated, multi barrel, recoil operated, electrically operated varieties. Today, most of the modern guns use multiple technologies borrowing optimized designs from each principle to have a high performance weapon. For this project, only blow back based systems were covered. However, it serves as a module to aid understand the design of guns is a broader sense; the design methodology for the other kinds of guns is just similar. In conclusion it can be said that gun design is a fairly complicated task, requiring expertise in mechanics, materials and safety engineering. Finally to end in a note of peace, we recall Martin Amin’s words I I T G a n d h i n a g a r!


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“Bullets cannot be recalled. They cannot be uninvented. But they can be taken out of the gun.” -Martin Amis Guns are just for self defense, they are not meant to kill. If no one hates anyone, guns would be unnecessary. The study of their design, yet remains as involving and interesting as ever.



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Bibliography [1]!

D. Grossman. (2000). Evolution of Weaponry.

[2]!

G. M. Chinn, The Machine Gun: History, Evolution and Development of Manual, Automatic andAirborne Repeating Weapons vol. 1: Department of U.S. Navy, 1951.

[3]!

G. M. Chinn, The Machine Gun:Design Analysis of Automatic Firing Mechanisms and Related Components vol. 4: The Department of U.S. Navy, 1955.

[4]!

D. L. V. D. Brink. (2007, 29/06/2012). Catastrophic Failure of Rolling Block Rifles.



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Design of Automatic Weapons_Puneeth_abhik

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