its a tapered fluting. it goes rather deep into the barrel, starts small, and as you get closer to the muzzle, expands and less and less of the barrel is at bore size, most is relieved by the flutes.
i keep referring to the sound, because that was very striking. like an old crownpoint, it rings like a bell. clear, and for a long time. PING. PING. its certainly unique to shoot.
i thought it was fascinating how efficient it was. it didnt improve accuracy with regular paint, but it was more efficient than one would think based on its bore size and the amount of porting. there is certainly something to it ...