^ What he said.
Ammo manufacturers usually list their test barrel lengths on their website with the load specs. That being said, their velocity specs are only true to the barrel they used, to the ambient temperature, elevation, and several other external factors at which the rounds were tested.
In any case, difference in muzzle velocity between common barrel lengths is negligible as it applies to practical shooting purposes. You will likely experience as much variation in muzzle velocity using the same ammo in the same rifle on two days with varied temperatures. Only way to know for sure is to invest in a chronograph and compile your own data...