MPT 76

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Abstract

There are many sources, articles, books and documents in literature about weapons and their design. About weapon design there are less sources and all definitions are in general terms like mini skirt: they give you good ideas but hide the most important parts. In this article I want to tell you a story about our journey in design process of a military rifle or a machine gun in 7.62x51 mm caliber. And of course I never give or mention about the vital side of critical technical issues. Because in all other documents it is very easy to find necessary technical expressions and terms. But I strongly believe, this document would be a better guide to who wants to understand the design process of a small (light) weapon system.



INTRODUCTION

According to history of mankind and warfare, the definition of firing arms were always varied by the end user’s demand. After throwing stone, stick, blowpipe, javelin. Sword and at last arrow meant something about technical issues on payload. Payload is the magical word that is related with your aim (doctrine). Also the aim (doctrine) is taking down the enemy soldier, penetrate the armor, etc. Especially after invention of black (gun) powder, a standard definition became as minimum 80 kilojoule energy needed in 100 meter range for taking down an enemy soldier. This was the definition of ammunition in the same time. Ammunition was the main concern and its design came first. Weapon or gun or barrel (tube) will be designed based upon the ammunition’s desired specifications. With respect to this now we have more complicated terms about ammunition performance like described in STANAG 2310 as effective range of NATO SMALL ARMS AMMUNITION 7.62 bullet.

In many common sources, the sequence of weapon design is given as:

1. Ensure that the doctrine (i.e. proposed conditions of use) comply with the doctrine being used by the soldier.

2. To specify the target that must be defeated.

3. To specify the protection behind which the target must be defeated (body armor, physical protection from sandbags, etc.).

4. To specify the required target effect.

5. To specify the range of the target.

6. Design a projectile and round to meet the specification.

7. Design a weapon to fire the specified round.

According to the sequence above you always need a concrete doctrine. Within doctrine you have the knowledge about the concept of infantry weapons. Therefore there may be a very serious survey study before your design studies should be done.



SURVEY STUDY

Today there are more than 185 different type of military rifles with 7.62 caliber are widely using in modern armies in the world. The doctrine is almost equal in all armies. Surveying the existing weapons is one of the finest method to understand the concept. Because you have to make some decision about:

 Operating Mechanism

 Blow Back Operation

 Recoil Operation

 Gas Operation

Surveying of existing systems may give you some hints about your road map. In our survey study we observed that the most weapon systems were excellent in many aspects according to the defined battle volume; urban warfare, conventional warfare, jungle warfare, shore and maritime warfare, etc. In some cases the end user wants more accuracy and consistency, in the others high reliability or easy maintainability. But most commonly easy maintainability and endurance under extreme environmental conditions are becoming first for infantry weapon systems. Weapons systems should be flexible with using many special accessories like different type of optical sights, grenade launcher or shotguns using piccatiny rail interface in the fore body. The end users generally wants the weapon system like a Swiss Army Knife. Practically it means nothing to the designer. The main idea of the weapon should be solid and simple or plain for optimum result. For last 60 years in weapon sector the simplest design with simple technologies survived and widely used in the world like AK-47 or FAL rifles.

There may be some other considerations about your design such as; material of parts, weight of system, ergonomics. By your survey study you can make some preliminary decisions for all titles above. The next step will called CAM – Computer Aided Modelling. By this step you start the prototype studies.



PROTOTYPE

According to your survey study, preliminary decisions and calculations, you can create 3-D models in a proper computer tool like CATIA, AutoDESK INVENTOR and check the mechanisms and also preproduction in 3-D Printers. Solid models and 3-D molded parts from 3-D printers could verify your mechanism design in machine dynamics aspect but limited.

In this phase everything is theoretical and there is no physical or dynamic verification yet. Especially the springs that could be used in the mechanism are still undefined and unknown.

Also you may need a person who should be a material or metallurgy expert in your design team for selection of materials will be used in your design. At this phase the manufacturing processes of prototype arts are always hard. Especially for chip removal operations in CNC lathes you have to determine the dimension tolerances. The best method is usually choose a known or most common parts in literature such as mechanism body for starting. The other most known method is examined the existing mechanisms and their working characteristics, then applying trial & error method.

One of the most critical issue is setting the Cartridge Head Space as shown in figure below.

You will observe that there is no proper calculation for C.H.S parameter than experience. According to critical part prototype manufacturing, there may be some premature firing tests. Most of the time in that phase, you may not have all parts for completing a firing mechanism of a real rifle. You may cannibalised some complementary assemblies from a well-known weapon system including a standart barrel.

Premature firing tests would be much more vital for your design purposes as you supposed. It would be better if your test site is well-equipped with high speed camera, recorder, transducers, measurement devices as much as possible. If there is a professional and accredited ballistics laboratory this makes the trial time shorter.

In firing tests you should observe:

- Mechanism movement

- Extraction of shell

- Gas piston system movement

- Automatic feeder mechanism

- Mechanism springs

- Measure the pressure and temperature along tube (barrel) and at the end of tube (barrel)

- Use thermal and high speed camera positioning at lower body of weapon, along barrel and at the end of tube (barrel) if number of devices and capacity of measuring system is sufficient

According to your premature firing tests you may observe some performance parameters previously. By this you can assume your weapon abilities before official performance tests. At the same time you can collect valuable data for your manufacturing techniques for future. Mass production of a weapon is another main issue you may face with in next phases.

Beside the items above you would prove the material selection for weapon parts. By literature all kind of materials specified for each part are known. Your design should be depend very simple and common materials like AK-47 for wide use. Especially some alloys of steel and aluminium could only presented to market in major manufacturer countries such as Germany, Russia or China. Designer should consider this vital item at the beginning of design process.

In prototype phase if possible you do not let your consumer interfere the process. Your main goal is you have a design, you build and see if it works as designed.

The usage of computer tools are mostly in this phase. Because you have to support or change your design details the computer tool’s analysis results with respect to premature firing tests.



ANALYSIS

The main problem with tests and computer analysis tools is always cost. Computer tools are always expensive while purchasing, licensing and training the user phases. Equipped tests are already expensive especially including some computer tools, equipment and trained users.

The computer analysis tools can be given as below:

- Solid Modelling Tool

- Ballistic Analysis Tool

- Computational Fluid Dynamics (CFD) Analysis Tool

- Dynamic Analysis Tool

- Mechanical Design (Structural) Analysis Tool

There are many computer tools in use and known for purposes above. None of them is cheap and easy to use. All the tools should be used in order because the inputs and outputs are common for all. For weapon design all tools would be needed and 500K US$ budget should be concerned. At this point we may mention about total budget for design of a military infantry weapon system would be varied between 10M to 100M US$. For time needed for design would be never less than 36 months under ideal conditions.

According to analysis from computer tools you may have some results about your design. But all the time you need to support or deny the results by firing or another technical tests and measurements. Without this verifying process, analysis would be meaningless. This is the most important thing all along the design process of weapon system.

We may say something on the methodology along the order of analysis tools as; everything started with modelling tool with dimensions. With dimensions you need to calculate or produce tolerances in dimensions. It is also provide important data for chip removal or cast manufacturing procedures. In this phase you also add the material mechanical properties as density, modulus of elasticity, Poisson’s ratio, tensile strength and yield strength.

In next phase Ballistic Analysis or CFD Tools would be used. These two analysis tools are most common in inputs and outputs. In this phase you start design with ammunition specifications and move along the doctrine defined for your weapon system as range and capacity.

Basically you have a set of input like below:

 Effected Range

 Rounds per Minute

 Initial Velocity

 Tube (Barrel) Life depending of total number of shots fired

 Recoil

 Trigger Force

 Dispersion at Given Range

 Length of Tube (Barrel)

After calculations you have to reach initial parameters by converging using ballistic formulations. This method is verify your assumptions.

Dynamic analysis of whole weapon mechanism should be done coordinated with ballistic and CFD analysis. This is another vital analysis for design process. You can calculate the rounds per minute in machine dynamics term, displacements of mechanisms and durations.

Mechanical (Structural) Design is a complementary analysis and preliminary design might be ended at this phase. Also all the parameters are calculated, measured and verified by firing tests in the equipped ranges and polygons up to this stage.



CONCLUSION

After analysis and completion of prototype your platform may be ready to performance tests according to related military standards. This test shows your design will be cover all necessary demands in customer side. For a 7.62x51mm military infantry rifle there are more than forty different tests in literature. Total duration of those tests will take between six months to two years at least. For H&K-19 the test procedure is still ongoing for more than ten years as known.

Completing prototype and performance test process, there will be another vital topic as mass production of the weapon system. Before this phase you have to complete your manufacturing drawing of your product with all material specifications, production methods and geometric tolerances of dimensions in engineering drawings. Mass production of a weapon platform is another important issue and will be mentioned in a further article.

Cagri Dogal Gul

Mechanical Engineer, BSc.