Removing metal from a barrel cannot make the remaining barrel stiffer, only weaker. As for heat dispersion, it must greatly increase the surface area. A flute that was milled to look like a spline or radiator fins would accomplish this, but four to six flutes don't achieve much. In fact, if the inverted arc of the flute closely matches the arc of the unfluted portion of barrel, there is a zero gain to the surface area.