Of course a longer barrel doesn't ALWAYS offer more velocity. To clarify, there are three forces on the bullet: 1) pressure force (pressure x bearing surface) from the combustion gas behind it 2) pressure force from the normal atmosphere in front of it (very small compared to peak chamber pressure) 3) friction with the barrel. As long as 1) is bigger than 2) or 3), the bullet keeps accelerating. As the bullet moves forward, the space in the barrel behind it increases, causing the pressure for 1) to drop. When it gets to be less than 2)+3), the bullet will decelerate rather than accelerate. The whole point of Quickload is to accurately simulate this process so we don't have to wave our hands and guess about it. And having done so, we can answer the first of OP's questions: you'll gain about 50 ft/s with a 180gr bullet going from 16" to 20". Whether you feel that 50 ft/s matters is a separate question.