I think that you would lose approximately 20 fps per inch. Personally, I like magnum cartridges to have barrel lengths of least 24" or more, and standard cartridges to have barrel lengths of at least 22" or more. My own personal exception is, say, the .45-70 in a Marlin Guide gun (which has an 18" barrel), but even then I still prefer longer barrels. The muzzle blast would be much more noticeable with those really short barrels.
Zachary