Fluted Barrel
A bull-profile barrel with grooves machined lengthwise along the outside. Cuts weight without giving up much stiffness, and speeds heat dissipation.
Fluting trades a small amount of barrel stiffness for noticeable weight reduction and faster cooling between strings. The flutes increase surface area, so the barrel sheds heat into the air more efficiently.
The aesthetics are also nice. A fluted Bartlein or Krieger looks like what it is — a precision instrument.