Originally Posted by jismail
Is this due to the longer barrel producing more 'burn time' for the powder thus developing more pressure for the same round?
I believe it's more a matter of how long the compressed combustion gases are able to push against the bullet. Once the bullet leaves the barrel it's no longer subjected to that pressure. Longer barrel = longer push.
For a cartridge that expels a lot of unburned powder from a pistol barrel (i.e. lots of muzzle flash) a rifle barrel may allow time for the powder to burn more completely, which will help increase velocity too.