A typical handgun round (9x19, .40, .45 etc.) doesn't gain much with barrels over 5-6 inches. The average gain for both 9mm and .45 in my own testing going from 5" to 16" barrels was between 100 and 200 FPS, depending on bullet weight and charge weight/type. My guess is that most of the addional velocity was achieved between 5 and 10 inches, with the last 6 doing very little.
OTOH, big rounds like the .50 AE or .500 S&W burning 30+ grains of powder behind large, heavy bullets will benefit considerably in a rifle-length barrel.