To add a bit to Mikey's comments: Most of the published data for muzzle velocity & energy is from a 26" barrel.
Wile it varies from cartridge to cartridge, a rule of thumb is a loss of from 60 ft/sec to 100 ft/sec per inch of barrel less than 26".
It is my understanding--and I could be wrong--that cartridges such as the .308 and the .243 suffer less than, say, the .30-'06. The reason seems to be the width vs. length relationship of the case. Extrapolating from this, then, it would seem that in general a Short Fat Fannie sort of cartridge case would have less velocity loss than a Long Lanky Louise...
(You're on your own, now, Bubba.)
I vaguely recall an article in the Rifleman discussing testing where a 30" barrel produced no notable gain in velocity over a 26" barrel.
Having nothing to do with your question: Short barrels are LOUDer...
In the FWIW department, shifting to pistols, an experiment with a 44 Maggie started with a 10" or so barrel. The usual results occurred, for each inch of reduction. The final chronograph, just for fun, was with the barrel unscrewed from the revolver. The "muzzle" velocity through the chrono with a 240-grain bullet was around 1,100 ft/sec. Barrel? Who needs barrels?
, Art