Well the longer the inner barrel is, the more it stabilizes the projectile. Simple physics there.
The more stable it is, the farther & straighter it will fly. The velocity is just as important, as it will send the projectile further and faster.
I'm just wondering why it doesn't have a calculator for barrel lengths. As it seems the answer's provided are on the basis that all inner barrel's are of the same length, or so that is how I'm perceiving the data that is being presented.
What's your thought's on that?