It should be noted that most pistol cartridges don't gain very much velocity from the longer barrels, especially since max velocity will usually be achieved with a barrel around 8" in length.  Longer than that and the pressure spike will have dropped off and barrel friction will be slowing the bullet down again.  Remember that these rounds are loaded with powders designed around 4" barrels.

Larger-capacity cases, such as most revolver cartridges, can be loaded with slightly slower powders for use in rifles, but those cartridges should not be used in handguns.

-Troy

Quoted: Troy, Why not? Why can't use ,say, a .357 Mag cartridge loaded with rifle powder in a long barreled revolver? Thanks CRC

Well, for one, you can generally load the ammo meant for rifles to higher pressures, pressures that would kB! a handgun.  Second, unless you had a REALLY long pistol barrel, you probably wouldn't see much of a gain.  Remember that normal pistol ammo maxes at around 8".  Not many revolvers with longer barrels than that.

-Troy