The HAMR fires from the closed-bolt position on both semi-auto and full auto, and automatically transitions into the open-bolt position during both fire modes (semi-auto and full-auto fire), once the chamber reaches a certain temperature (like during sustained fire situations), in order to prevent cook-offs. It then automatically transitions back to closed-bolt operation for both fire modes when the chamber temp drops back down to a safe level, making the FN HAMR an automatically thermo-regulated firearm/infantry small arm. That’s interesting.
Previous IARs just transition from closed-bolt semi-auto to open-bolt full-auto, once you hit the happy switch (safety/selector-switch) over to rock n’ roll (full-auto setting). While the primary purpose of switching to open-bolt operation for full-auto fire (and/or semi-auto fire, in this case) is to prevent cook-offs (rounds firing without pulling the trigger, due to excessive chamber heat) after extensive full-auto fire, open-bolt operation is required for certain types of weapon recoil
