ttp://www.fnhusa.com/l/products/scar-family/hamr/
>The FN HAMR is designed to operate in either the closed-bolt mode or the open-bolt mode using a dual-mode firing mechanism that gauges the barrel temperature.
>Switching from one mode to the other is automatically performed by the thermal actuator located under the barrel just forward of the chamber.
>This allows the FN HAMR to instantly transition from closed-bolt mode to open-bolt mode and continue to operate safely
>even when barrel temperature has reached the cook-off point. When firing stops and the barrel cools,
>the thermal actuator self-resets and the FN HAMR switches back to the closed-bolt mode, all without manual interventionof the operator.
>Closed-bolt accuracy and open-bolt firepower in one weapon are now a reality with the FN HAMR.

サーマルアクチュエータはこういうの。
ttp://www.thermalactuators.com/tech_data-thermal_actuators.html
要するにバレルとチャンバーの熱量が一定超えたらアクチュエータ起動してボルトが後退位置で停止するようシアーと連動、
熱が下がったら通常通りのクローズドボルト動作に戻るっていうシステムかと。