This is my initial attempt at establishing a formulated means:

(LB2/LB1)^(1/3)*VB1 = VB2

Where:

LB1 = Effective Length of Barrel 1 in inches

LB2 = Effective Length of Barrel 2 in inches

^ = Raised to the exponent power of

* = multiplied by

VB1 = Velocity measured in Barrel 1

VB2 = Velocity estimated for Barrel 2

NOTE: The exponent value of 1/3 as seen here is merely an initial estimate. This exponent value can be varied whereby to hone in upon greater precision.

NOTE #2: Effective Barrel Length is measured from the crimp position of the shell within the chamber to the muzzle.

It doesn't work that way, as in direct proportion to barrel length. There are some

*generalized observations* made over the years concerning certain cartridges and the effect of barrel length, but they are all just that. 6 fps/inch for shotguns. 25fps/inch for .223 (whatever it actually is, this is just an example for argument sake), but that changes dependent on exactly how long the barrel is.

Additionally, longer barrels don't always increase velocity. Powder changes are required when you get to extra long barrels. For example, I have one .22, 18" barrel that is about 80fps longer than my other .22 with a 24" barrel.