I've read articles on this subject as well, and they are all very interesting. I'm surprised no one has written one as of late, because there is quite a bit that has changed since these ideas were floating around.
The article I read also referred to the length as what most would call the "control bore" these days (ie: length before the step or porting). According to this article, which I believe was in an APG back in the early to mid-90's, the most efficient barrel (not accurate) was in the 8-in range. Keep in mind that most markers of that day were operating on either straight CO2 (between 600 and 900psi) or mags that were regulated to operate between 600 and 700psi. The higher pressures could push a paintball up to 300fps in a shorter distance, so it was determined that 8-inches was about all that was necessary.
It also mentioned that markers operating on lower pressures (ie: autocockers running between 400-500psi) would require a slightly longer barrel to get the required velocity without a significant loss of efficiency. They stated that 10"-12" was best suited for these markers.
Classic Mags had stock barrels of 8-inches, Cockers came with 10-11 inch barrels, and PPS always keeps their markers in that same range of barrel lengths. But anymore, people are dropping the "expandable" CO2 for a more steady state compressed air, which debunks a lot of that theory. They are also less interested with efficiency and more interested in speed.
I agree with Sniper1rfa above. CO2 will continue to expand to the end of the barrel in a properly tuned marker and can maintain (if not slightly increase) the velocity for a longer distance than compressed air or CO2 that has already expanded completely before the barrel ends.
There are a lot of factors involved anymore. Anyone up to the challenge?
Bleeding Paint and Sweating CO2 since 1988!