Take a look in your reloading manuals. Most manuals tell you on the front page of the cartridge you are loading for what firearm the results were taken in.
Many were taken in barrel lengths longer than the standard hunting rifle. Some debate the point, but you can GENERALLY take about 25-30 fps. MV off for each inch of barrel your gun is shy of the test firearm. Again, some of us question the published MV figures from the factory for their creations to begin with............