The MK.17-M110- A stronger foundation for the SCAR

The MK.17-M110- A stronger foundation for the SCAR 

Drag racers always look to put the strongest motor in the lightest car. Light weight and big power leads to better performance. But often racers put motors in cars that were not designed for it. The engineering analysis of the manufacturer was for how the car was originally designed. These cars could be so overpowered that engine torque would overwhelm the frame to the point where the windshields would crack.

We find some similarities to the design of the SCAR®. The SCAR® was designed as a 5.56x45mm gun first. The 7.62×51 SCAR® Mk.17 shares a lot of the same materials and parts. The SCAR® Mk.17 was not the main effort, the SCAR® Mk.16 was. Since USSOCOM had a proven and well liked 5.56×45 gun already, the SCAR® Mk.16 was dropped. The SCAR® Mk.17 became the main effort. FN Herstal put a big motor in a small car and we went to war with it. I have been a fan of the gun ever since.

Since Handl Defense seeks to extract the maximum performance out of the system, just like racers do. We found some similarities in solutions to what drag racers would do to reduce flex. Why do you want to reduce flex and vibration in a gun? Let us start with more durability and accuracy. In a rifle, you have areas in the design that overall are more stable. If you remember your physics, these are nodes. The barrel extension is place of high stability in a SCAR® and therefore a node.

Areas that have less stability along an axis are called anti-nodes. Examples are the end of the barrel and places along the track of the operating group. These oscillating and flexing areas, like the barrel, are what cause harmonic vibrations. This effects accuracy in a negative manner. Rigidity reduces the amplitude of these vibrations and is therefore equivalent to mechanical accuracy. Discussions about barrel profile, tuning, timing, and dissipating the now increased frequency of the small oscillations will be for another time. But to better understand nodes and anti-nodes in an alloy, look at this video.

Where does Handl Defense add extra rigidity in the SCAR®? All along the long axis of the platform. We also reduce torsional, longitudinal, and shear forces of the weapon as well. We do this in several ways.

First, is tighter fitment of the trigger module. Our trigger modules were originally designed to require press fit by rubber mallet. Once the trigger module was mated with the upper receiver and after a break period of about 100 rounds the two components would wear together in a way that would result in a long term tight fit. This tight fit kept the nodes and anti-nodes of the energy waves in effectively the same places. It also reduced the amplitude of these waves along the entire track of the operating group.

This design theory caused complaints in the early models of the SCAR25. The longitudinal tension was decreased in subsequent versions of the SCAR25 and MK.17-M110. Handl Defense still retains the stressed member concept in our current trigger modules. We are the only SCAR® trigger module producer who understands and applies these concepts.

Inversely from a tight-fitting set of components, a loose-fitting set of components will also adversely affect mechanical properties. Nodes and anti-nodes will change locations along the long axis of the firearm the more flex is introduced to the platform. Harmonic wave patterns can even superimpose on each other. Where these waves superimpose, a new pattern emerges with even greater amplitude known as constructive interference.

With the addition of the Handl Defense MK17-M110 series of trigger modules, we provide more structural rigidity to the platform. Handl Defense has provided substantial increases in the platforms compressive strength and torsional rigidity. Now we provide much stronger support for the entire chassis. This is important as the extrusion profile lends itself to vibration.

Another way we make the system more rigid is to use different materials. Imagine two objects of the same weight dropped from the same height on the same surface. These two objects are of the same size and shape, their only difference is the one has high elasticity and the other does not.

When the highly elastic one is dropped, it bounces and comes back almost as high as it was dropped from. What happened is the potential energy was converted into kinetic energy that was transmitted back into the ball due to elasticity.

When the inelastic ball is dropped, it drops on to the surface and does not bounce. This potential energy was converted to internal energy back into the ball and the surface. This is caused by the inelastic nature of the collision.

To simplify, elastic collisions maintain momentum (bounce/ vibrate) and inelastic collisions dissipate energy into the objects themselves. This is consistent with the conservation of momentum. Which means the energy must go somewhere. Into something else or back into itself. Materials that have a lower tensile strength will vibrate longer and with more amplitude (larger waves).

To understand the amount of energy that is dissipated into a material we need to understand Young’s modulus. Which on a basic level is stress divided by strain. Stress is the force applied to an area. Strain is the elongation of the material over the original length.

One way to measure strain is tensile strength measured in megapascals (mPa). A pascal is one newton per square meter, one megapascal is 1,000,000 pascals. One mega pascal (mPa) is 145 psi. The yield tensile of 7075-T6 Aluminum in the Mk.17-M110 is 500 mPa. The AZ 31 in the Mk.17-M110 MOD 2 is 220 mPa. The SCAR® upper receiver is a 6000 series material. The yield tensile of 6061-T6 is 145 mPa. With twice the stretch at failure of 7075-T6. The polymer we believe to be a urea formaldehyde, cellulose filled polymer with a probable mPa of about 55.

In OEM configuration, the SCAR® upper receiver takes all the forces of the cycle of operation on its own. The upper receiver is a homogenous extrusion with a tensile strength of about 145 mPa. It is also much stronger material than the trigger module. By the nature of the design there is no ability to transmit any of the operating forces to the polymer. When recoil waves pass through each other without being disturbed, and where they meet, the net amplitude is the sum of the two individual waves. This constructive interference is known to wreak havoc on sensitive optics. 

When a Handl Defense trigger module is mated to the SCAR® the trigger module fitment and material properties effect the upper receiver harmonics. It now has active resistance at the receiver end plate and barrel extension that will not allow for normal 7.62×51 recoil pattern amplitude in 6000 series Aluminum. This is cancelled by the much stronger 7075-T6 or AZ 31 trigger modules. The energy patterns are dampened by these materials. We specifically designed the MK.17 series of trigger modules to have destructive interference. Which means that energy waves moving through the trigger module and upper receiver are moving in very different patterns. This destructive interference allows for more nodes to develop along the pathway of the operating group. When the energy waves of the FN SCAR® upper receiver and the materials of Handl Defense trigger modules have opposite phases, they help cancel each other.

In the diagram below if wave A is recoil forces moving in the SCAR® upper receiver. Then wave B is recoil forces headed in the opposite direction. Since the material is homogenous there is no external interference. When these recoil waves meet they add to each other. This constructive interference is 3700 joules of energy for 7.62×51 instead of 1800 joules for 5.56×45 the SCAR receiver was orignally designed for.

The next diagram wave A represents the recoil forces in the upper receiver. Wave B represents recoil forces in a Handl Defense trigger module. Due to the stressed member concept specific to Handl Defense, recoil forces are transmitted into the trigger modules. Handl Defense trigger modules are made of very different materials, chosen for their higher tensile strengths. Recoil forces are transmitted in a different frequency. This different frequency results in destructive interference, reducing recoil forces transmitted throughout the platform and into sensitive accessories.

Understand Handl Defense came to these conclusions without any assistance from FN, USSOCOM, or NSWC Crane. We have never received input from any of those organizations, ever. We have sought our own solutions and forged our own pathway. We in effect did what the racers did, we reinforced the frame by providing stiffer support and selecting specific materials. We do what racers have always done, whatever it takes in the name of better performance.

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