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"I guess that blast out the side doesn't reduce the pressure completely.'
Right. Something from an engineering point of view, the high presure of the propellant gas is also quite dense. That means the forward flow of the dense, high speed gas keeps it sorta "focused" on the base of the bullet. It has a harder time turning to exit the cylinger gap that it first appears.
How much velocity we gain or lose per inch of barrel depends a LOT on the load itself; the time/pressure curve makes a difference. Generally speaking, we gain more speed with long barrels if we use a slower powder, while short barrels loose less speed per inch of barrel if we use faster burning powders.
That said, there is a common perception that we should get higher speeds from short barrels if we use faster powders; that's not true. The powder that gives the best velocity with any given bullet will do so reguardless of the lenght of the barrel. But, the ear-splitting muzzle blast of loads with slow powders fired from short barrels is much higher because the bore pressure at the moment of bullet exit is higher. Again, it's the time/pressure curve of the load that counts.
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