In my experience, a barrels velocity potential has very little to do with its bore diameter alone. There are many factors to figure into the equation and so many variables that its nearly impossible to predict which barrel will be faster then another.
Things such as number of lands in the bore, wideth of the lands themselves, throat diameter, throat length, lead angle, all of these things will combine to result in the velocity of a barrel far more then just the diameter of a bore.
Of course bore surface quality also comes into play as well.
I have seen exactly the same barrel design from a top end barrel maker produce nearly 100 fps difference in top velocity potential.
To figure out exactly why this happens is more likely to give you grey hair then actually conclusions because your theory in most cases will be weakened with every barrel you test down the road.
I have also found that actual twist rate will effect velocity potential very little comparing slower twist rates with faster twist rates. That said, the most dramatic differences I have tested with velocity change in relation to twist rate has been with some of my more extreme Allen Magnum wildcats.
I should correct that statement by saying, pressures are effected more by twist rate in my opinion moreso then velocity is. This is what I mean by that.
In my 7mm AM, I can take a very long 200 gr ULD RBBT in a 1-7 twist 3 groove barrel that is 30" long and get 3300 fps comfortably with 102 to 103 grains of WC872 on average.
I can also take a Lilja 1-9 twist 6 groove barrel, also 30" and get that same 3300 fps just as comfortably but it will take 105 to 106 grains of the same powder to reach this velocity.
What this tells me is that while the twist rate does not really seem to limit the velocity potential of this combination as far as fps produced, the twist rate does effect how many grains of powder are needed to get X fps.
To be really fair, this comparision should be with either two 3 groove barrels or two 6 groove barrels, not one of each!!
Still, in my opinion, bore diameter does very little to effect velocity potential in and of itself.
Kirby Allen(50)