The flutes are an outgrowth of the original G3, and there for the same reason. Rifles that have a bolt that turns have what is known as “primary extraction.” The act of the bolt turning breaks the fired case free of the chamber walls, to one degree or another. This makes extraction easier. The roller-lock system does not have primary extraction. The bolt moves straight back. With a 9mm that might not be a big deal. But with the G3 and .308, it was. So, the chamber was made with flutes in it. Each flute allowed a small amount of gas to escape back (once the bullet has left the case mouth) and “float” the case off of the chamber walls. Those are the source of the black scorch marks…
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Firearms News (Digital) - 1 Issue, Volume 75, Issue 9