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For those of you up to date on your firearms news you’ve undoubtedly heard of the recent announcement that FN Herstal is discontinuing (mostly) the SCAR series of rifles. If you haven’t heard then I hate to be the one to tell you. The beloved short-stroke legends of SOCOM are now waning into their twilight years.With that in mind, I wanted to take this as an opportunity to take a look back and shine a light onto some of the SCAR programs highlights, and in this case in particular, one of its known but often-times less acknowledged rifles. The FN HAMR-16.
The HAMR may hold the outward appearance of a SCAR which is unsurprising, it is family after all. In function however it’s anything but. FNs entry into the US IAR program, the FN Heat Adaptive Modular Rifle, or HAMR is a Light Automatic Rifle/Light Machine gun designed in 2008 and making its first major public appearance at AUSA2010. It became one of Four IAR finalists in the USMC IAR program, eventually losing to the H&K M27 IAR. An ordeal I’ll explore in detail in the future. It is at its core similar to the FN SCAR the major difference setting it apart being its unique ability to adapt its firing mechanism to the chamber temperature of the weapon.
In its default cold state the HAMR fires from the typical, and expected closed bolt. As the rifle heats up due to firing it will transition to firing from the open bolt without any input from the user, likewise returning to closed bolt when sufficiently cooled. This all occurs mechanically and with no input from the user.
Design Details
Upon initial examination of the HAMR one will find very few differences, the first to jump out at a keen eye would be the enlarged lower rail, accompanied by a mysterious protuberance. This is where the bimetallic thermal actuator lies. Interfacing directly on a long linkage riding on the outside of the lower receiver, leading directly to the fire control group. This along with the typically black, but sometimes standard FDE lower are the most striking visual differences. Other immediately noticeable features are the HAMRs significantly heavier barrel, and extra screws at the rear of the receivers.
While the Marine Corps specified design requirements for the new IAR, FN also had their own in-house requirements for their submission. Wanting to take advantage of the modular, and by then well-defined system architecture of the SCAR, FN set out to redesign the lower rail/heat shield. Reworking its design to incorporate the thermal actuator. The trigger frame would also be altered to integrate the new fire control module. The receiver would see various alterations, and reinforcement, as well as the addition of a long mechanical linkage from the Actuator to the fire control module.
In regards to the Thermal actuator it by itself had its own strict requirements. It needed to engage, and disengage at an accurate, consistent, and well defined temperature. That temperature being determined by the internal temperature of the system, unaffected to a significant degree by external temperature of the firearm. Ultimately and perhaps more challenging, it would have no more than 4 separate parts. The Housing, Piston, seal, and phase change material. The housing, as you can see, is mounted directly onto the barrel just forward of the chamber. This position was determined based on finite element thermal analysis, and temperature mapping of the system during firing. Primarily though probing an experimentation with thermocouples mounted directly to the barrel, chamber, and actuator housing. Within the housing a piston resides with a contained thermo-expansive compound sealed in the cylinder.
As the rifle fires, and heat conducts into the metallic body of the actuator, the compound heats up expanding and pushing out the piston. This piston then expands outward, interfacing on the linkage bar connecting it to the fire control module. When the piston has extended sufficiently the mechanism trips a new sear, moving it into position, and automatically engaging the Open-bolt operation.
While doing my research for this i managed to get the contact information of one of the people involved with the second generation HAMR project, while he didn’t specify exactly what the compound inside the piston was, it was stated that the mechanism was a “Bimetallic” Thermo-Actuator. Which leads me to the belief that the compound is some form of metal or alloy with a quite low melting point.
During testing of the first generation of the HAMR the rifle seemingly managed to meet all requirements set both internally by FN, as well as externally by the IAR request. While the HAMR ultimately wouldn’t be selected for the USMC IAR contract, it wouldn’t be deemed a failure by FN as they continued development. Having met expectations, and boasting “The accuracy of a top class assault rifle, and the firing schedule of an LMG all in a single gun,” the HAMR went on to a second generation of development. A quote that does effectively summarize its performance. According to the NDIA Small Arms Systems Symposium, Exhibition and Firing Demonstration in May of 2011, the first generation MK16 HAMR was capable of 2 MOA accuracy whilst firing M855, and “close to” 1 MOA while firing MK262 Mod 1. Admirable accuracy for any combat rifle, let alone a light machine gun. The second generation would pursue the same success with accuracy, and firing schedule, as well as a new goal. A greater than 100 round capacity.
Going forward with the second generation of HAMR rifles, they would begin to be seen showcasing a large Beta C-Mag style magazine for the reason of boosting onboard ammunition capacity. The large grey double drums would be seen with several of the rifles at various exhibitions, and demonstrations, and continues to be seen occasionally. The second generation would see less drastic changes from the base rifle, instead focusing on further refinement of the overall system as a whole.
The HAMR didn’t win the IAR contract, but it wasn’t dead in the water afterward. It is still in development according to FN as of 2018, and has been seen, and is confirmed to still be in testing with FN as of 2019 albeit under the different name of the FN XLAR. At this point in time the HAMR remains as a 5.56 NATO only platform. It is possible that in the past however there was a 7.62 NATO consideration at some point in time though I was unable to find any reliable or trustworthy source speaking of such a thing. Though I've heard the name MK17 HARM repeated in a few different places.
Personal Thoughts
I can’t personally test the thermal-mitigation claims, but the concept is sound. Switching to open-bolt operation prevents the chambered round from being exposed to a dangerously hot chamber during long bursts. Eliminating the risk of a runaway gun or cook offs whilst simultaneously increasing air flow for cooling. That said, I imagine the abrupt change in firing mode, and the resulting shift in recoil impulse from closed to open bolt would feel odd, maybe even jarring. To a shooter caught off-guard the sudden change in firing may be disrupting to a degree. However in the hands of a gunner/rifleman experienced and used to such an idiosyncrasy I have little doubt it could be dealt with easily. It’s an interesting trade off, increased cooling and safety at the cost of a variable impulse under prolonged fire.
Still, the HAMR is the only notable automatic rifle to use this sort of self-adjusting bolt behavior, and that quirk gives it a special place in the hearts of firearms nerds and tech-minded shooters. Even if the SCAR is in its twilight years, the HAMR is a reminder that maybe sometimes asking yourself a weird question can lead to some seriously unique innovation. Like, what happens if the rifle can manage its own heat? Then engineering the hell out of an answer.
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