Pressure will be the same if your barrel is 14" or 28". What will be different is the velocity. The pressure spikes shortly after ignition but the force of the explosion "pushes" the bullet down the barrel. Now, if you have a short barrel, that force doesn't have long to push and thus, the bullet's velocity will be less than if it travels down a longer barrel....cause the force has a longer "tube" to push the bullet down, and this will increase the velocity of the bullet. (to a point. If you put a 8 foot barrel on a gun I don't think you would see much of a velocity change between that and a three foot barrel....but I am guessing at the lengths.).
Look at it like you were being pushed in a little red wagon. If someone were to push you for 4 feet and let you go vs. pushing you say, seven feet. You would be going faster being pushed 7 feet.
Dave.