...because the round was designed to burn all the powder in the first 9 inches.
I am pretty sure that is not correct. With some powders this occurs, but it is a function of the powder/pressure curve of the chosen powder, not the "design" of the cartridge. I am not advocating slower powders or faster powders, just pointing out that the "ideal" window is based on several factors. A 20" rifle length gas barrel can also suitably run 300BO, as could a blow-back 4" barrel if one was so inclined. There are people experimenting with 150 grain .30 caliber slugs at 2650 fps out of AR-15s with 18" barrels and significant changes in the BCG/gas system, so let's not shut the door yet.