well considering from the GGO arc even when a person's arm/leg was blown off they were still alive and such so I am guessing they went with the video game damage and also even Sinon had said even if they were hit in the leg/arm with her Hecate they would still be alive so that's a big flag that it definitely was not hardcore damage xD

Alos another thing a headshot is not the only 1HKO there were also heart shots