FN HAMR

When the US Marine Corps put out an RFP for companies to submit automatic rifle candidates, FN responded with a SCAR derivative that had some neat technological tricks. Let’s take a look.

The HAMR-16 (Heat Adaptive Modular Rifle) looks like the SCAR-16 that it was derived from at first glance. Same monolithic upper, same folding/telescoping stock with two-position cheek riser. Same polymer lower with short-throw (90 degrees of total travel) safe/semi-/full-auto selector. Same love-it-or-hate-it reciprocating charging handle.1 The barrel has a notably heavier profile than on a regular SCAR-16, and there is a heatsink protruding from under the handguard.

All of that might be expected to handle the sustained fire requirements of the IAR program. But FN hid an extra trick inside the HAMR. They put in a bimetallic thermocouple on the barrel, just in front of the chamber. As the barrel heats up from use, the thermocouple draws a linkage forward, activating a secondary sear. So when the gun got hot enough, it would automatically convert from closed-bolt to open-bolt operation. After the gun cooled, the thermocouple would push the linkage back, automatically returning the weapon to closed-bolt operation.

I might have suspected such a system to potentially cause problems, but the USMC 60,000 round reliability/endurance test showed otherwise. The goals for the IAR were as follows:

  • Three Units Under Test (UUTs) were provided for each model under evaluation.

  • The UUT shall have a Mean Rounds Between Failure (MRBF) of 900 for Class I and II failure combined (Threshold), 5,000 (Objective). The MRBF for Class III failures shall be 15,000 (Threshold), 20,000 (Objective).

The definitions for failure classes are as follows:

  • Class I failure: A failure that may be immediately corrected by the operator within 10 seconds or less while following prescribed immediate action procedures.
  • Class II failure: A failure that may be corrected by the operator, and that requires more than 10 seconds but not more than 10 minutes to correct (less the TM/OM defined cool down period if a hot barrel condition exists). Only the equipment and tools issued with the weapon may be used to correct the failure.
  • Class III failure: A failure of a severe nature. The failure (1) can be corrected by an operator but requires more than 10 minutes; (2) cannot be corrected by an operator and requires assistance (no time limit); or (3) requires higher level of maintenance or correction by an authorized operator cannot be accomplished because of unavailability of necessary tools, equipment, or parts.

The HAMR-16 met the objective goal of 5,000 mean rounds between class I and II failures, and was the only entrant to do so. It did not experience any class III failures, so MRBF for class III failures could not be computed. It was also the only entrant to not experience any class III failures.

For comparison, the winning HK entry that would become the M27 IAR had an MRBF for class I and II failures of 1,622, and a MRBF for class III failures of 20,000.

The HK entry was estimated to have a significantly longer barrel life than the HAMR. It was also somewhat lighter, with the HAMR weighing in a bit over ten pounds (unloaded and sans grip pod it’s usually shown with), and the M27 weighing a bit under eight pounds (unloaded and sans accessories). I might also expect the HAMR to be a bit more expensive than the M27.

I would have rated the HAMR better at being an automatic rifle, and the M27 better at being a backdoor carbine improvement, for what that’s worth.

As for my opinion of the project, I think it’s a very cool design with some well thought out innovations. However, I’m not a big fan of the automatic rifle concept, so I’d pass. It might be interesting to see what the thermocouple/sear setup does in a more standard SCAR design as far as reducing cook offs goes.


  1. Lots of people hate it, but SOCOM did request it in the original design. For what that’s worth. 

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