First off every barrel is certainly different as I found out, I have two Mark V SS 300 Wby's and my brother has one, all 3 rifles are identical, with Wby 180gr TSX ammo adv vel is 3240,
Leif's gun shoots 3280 fps
My 1st rifle shoots 3200 fps
My 2nd rifle shoots 3100 fps,
And that was out of the same box of ammo, velocity was tested with my labradar
I have two 240 Wby's 1 Vanguard S2 and a Mark V ULW, both rifles have 24" barrels, shooting 85gr TSX advertised vel 3500 fps
Vg s2 avg vel 3367
M5 ULW avg vel 3408
cartridges will have an optimum barrel length for velocity, a recent study on the 6.5 CM where they started out with a 26" barrel and cut back 1" at a time and found velocity increased until 23" then started to fall off after that.