Pressure in the case is what moves the bullet out the case. A case is designed to withstand only so much pressure. A certain length of barrel is required to burn the powder, not all is burned at the time of primer ignition. The bullet moves forward, contacts the lands and the pressure builds sufficently to start the bullet on it's journey. Bullet jump could enter into the equation, as in WBY, but we are still left with how much pressure is suitable for the system not how fast that pressure is made. With pistol cases, not short rifles, the case could not hold enough slow powder to build the pressure necessary to send the bullet at a reasonable velocity. If all we needed was a faster powder to use short barrels, our rifles wouldn't need the barrel length we commonly use to obtain our velocities. Barrel length allows the pressure to stay behind the bullet longer increasing the efficiency of the pressure created.
You use less of a faster burning powder to get the same pressure as more of a slower powder but you are dealing with the same amount of pressure. There is probably a barrel length where the faster burning powder might make a difference but it would be very short, think snub nose rifle.Rick.
