Let me put a twist(pun)
on this twist barrel, I am in the paintball industry of which I had apart in development of Rifle barrels for a company called Armson. Over the year we and many other companies tried different ideas and methods to control the ball . A round object is not aerodynamic , the PB or BB works the same as a old musket ball in a black power rifle , when the musket fires and the ball runs up the barrel and where it leaves the end tip of the barrel it will vear to so no control . The only way you can control the ball is putting a spin on to the ball , smooth bore barrels can be tight or large , the larger bore will let more gas escape around the ball when fired , while the tight ball while let lets air escape , thus the ball would travel further and need less power to move the ball.
If you over pressure any round object it will become unstable. Just the right amount of gas is need to get the a more accurate shot,sometime less is more. If you want to see the close thing to a bb shooting down a barrel Tom Kaye of Air Gun Designs shot some high speed footage usesing a clear barrel and you can see the ball shooting down the barrel. Look at
www.airgundesigns.com hope the footage is still there. Here is what we found to work that best on our design at Armson . Due to the ball getting made small we made our bore size about the same as the most avg ball which was a .685 ,the balls were in the range of .684,5 tru 9 diam so we kept it as close to the ball size for sealing, 2nd the was the length of the barrel , we found and in general that the best length was between 6 and 8 inches for acceleration of the ball and the total barrel length was 13 to 14inchs.We also found by drilling holes from the 9inch to the front part of the barrel ,the ball would be come stable and the ball would fly as stable as we could get it to fly and lastly we made the barrel with a progressive turn which started with a slow turn from the back a tighter turn to the front.
Just my cents