Suppressors and the SCAR part 1
I often tell industry insiders “I can cause a SCAR 17 to break itself whenever I want”. While this might seem like an indictment of the system, its not. It is an indictment of what the user puts on the SCAR. The Mk.17 is like a 370 pound, 225hp racing motorcycle, most will never find its limits. Yet, it is a tool for experienced and competent users. When you start messing with how it works, you just might break it. Without professional support, it just might be a matter of time until you crash and burn.
The easiest and most common way to mess with how a SCAR works is to attach a suppressor. FN only authorizes the use of one suppressor, the FN 701 on the Mk.17. The FN 701 is unavailable to civilians, but there are many commercial options for suppressors. It is imperative for SCAR owners to understand there are effective commercial market solutions. While not currently sanctioned by FN, it does exist. I am going to explore an ideal solution here.
Before we delve into the solutions we need to discuss suppressors and what they do. Suppressors stifle weapon signature such as flash, noise, and pressure. They do this by managing the gas expended by the cartridge once it has finished propelling the projectile down the barrel. This means we must understand gasses to understand how suppressors work. Once we understand how gasses work, then we can understand how SCARs are particularly affected by suppressors. Then we can decide what options are ideal.
To understand gasses in a general sense we need to understand three laws. First is Boyle’s law which is at a constant temperature relationship between the pressure and volume of a gas is proportional. Simply, if you add twice the gas you add twice the pressure. Second is Charles’ law that states at constant pressure, the volume of gas increases or decreases by the same factor as its temperature. When gas gets hot, it expands. Third, Lussac’s Law is pressure of a volume of a gas is directly proportional to the gas’s temperature. The hotter a gas gets the more pressure it exerts. So you can see that amount of gas, the pressure it exerts, and the heat are the three primary issues facing suppression.
The SCAR platform is extremely suppressor sensitive. Other systems are as well, such as the H&K G28/417/MR762 series of rifles. AR variants have very large and robust buffer systems. While the HK MR762 with a certain suppressor exceeded the G rating on our accelerometer, there was little discernable from the shooters perspective. This is due to the fact the HK MR762 buffer system is massive and the MR 762 is probably milled from the armor of Tiger tanks hidden in a bunker under Obendorf, Germany.
The SCAR on the other hand has a relatively small volume short stoke gas piston system. Remember Boyle’ law, more gas means equal increases in pressure. Then remember Charles’ law volume of gas increases under heat. Suppressors add both to a SCAR’s gas piston system in spades. This is a big deal. Remember when I said I could break a SCAR on demand. While I won’t give all the details, it involves a suppressor, and here is what happens.
When you put a non-optimal suppressor on a SCAR you can get bolt speed increases as high as an extra 50%. When viewed from a velocity squared times ½ mass perspective, that is huge. That is an immense amount of extra load on the operating system. With the bolt moving up to as much as 50% faster, you can easily exceed the design loads of the system.
I first noticed this when I was issued a Mk.17. We had just finished a combat rifle marksmanship course. I used my Mk.17 with a 13” barrel and a Suppressor. I noticed that the gun was running somewhat sluggish. It seemed to unlock from battery a bit slower and rougher than normal. What I also noticed is that I left the gun in an unsuppressed setting on the gas block. I had put unnecessary stress on the operating system by my own negligence. What I found is that I had made a burr at the very front of the inside of bolt cam pathway on the bolt carrier group. This was causing drag on the bolt and bolt cam pin during function. Some sanding and polishing during cleaning, I never had the problem again.
This event has always stuck with me. I remembered that this burr was where the thinnest part of the bolt carrier group is located. There have been a few commercial SCAR 17 bolt carrier groups splitting in this same location. This is not an indictment of the system. It is an indictment of what the user puts on the SCAR.
Let me explain how this is happening. The rifle is fired, the projectile passes the gas jet, gas begins to fill gas block, the suppressor also begins to fill. Due to the suppressor not being optimally designed for the SCAR, the gas in the barrel begins to back up on itself. Sometimes the gas is so backed up it is visible coming from the cartridge ejection port as the weapon operates.
The gas in the barrel moves to the path of least resistance, into the gas system. Remember Boyle’s law, more gas equals more pressure. The gas system works faster, harder and hotter than intended from the pressure. It pushes the bolt carrier group back much faster and harder than intended. The bolt, which is still locked into the chamber, provides for a sturdy support for the bolt cam. The bolt carrier’s accelerated rearward travel causes the thinnest part of the bolt carrier group, the bolt cam pathway, to slam into the high strength and well supported bolt cam. Instead of a smooth unlocking action, the action is more of a ripping the bolt from the chamber. The energy is moving to the path of least resistance.
Under enough load this might cause damage or in some cases failure of the bolt carrier group in and around the bolt cam pathway. Imagine not using the clutch in a car to change gears. It is the poorly integrated suppressor that causes the load to exceed the engineering of the system. Like I said velocity squared is the important part of the equation. This is a repeatable event under the right conditions with a particular set of items used with the SCAR.
FN designed the gun to work with certain parts, their parts. As a manufacturer I understand this perspective. Once a gun gets to its owner there is no way to control what they are going to do to it. Switching out gas jets to meet the gas flow of the suppressor is one option to the solution. This relies on a competent gunsmith, of which I will only recommend Alan Handl or Jerod Legslin for this type of work on a SCAR. Also, it only deals with the filling of the gas system issue. It does not relieve all the extra bolt speed and over pressure issues I’ve discussed. If you add heavy use or full auto fire into the equation, Boyle’s law becomes exceedingly relevant. Your results will vary, user beware.
All gas issues are only resolved by a suppressor optimized to work with the SCAR 17. One which efficiently evacuates all the gas, reduces back pressure, and minimizes bolt speed increase. One which was designed with optimized use on the SCAR17 in mind. One which has hundreds of thousands of rounds in testing on a SCAR. One that was built with assistance from SCAR experts directly involved in the design, testing, development, and feedback.
Next: Suppressors and the SCAR part 2: The contenders