I just ran the numbers and the Renolds number for a 6mm BB at 120m/s is about 48000. Which means that dimples will most likely have little effect. However it is entirely possible that my understanding of the problem is less that required to make that judgement. I do know that introducing dimples will make for less drag in the barrel but possibly a harder surface to polish.
I think a shaped projectile has more potential for a sniper type of round.