Case dimension mostly determines optimum burn rate. I have experimented with cutting rifle barrels off down to 20" and the same powders that provide maximum velocities with a certain cartridge at 24-26" also provide maximum velocities at 20". (And no, cutting a .300 Winchester Magnum down to 22" doesn't turn it into a .30-06. I cut a .300 WM to 22" one time and standard manual loads still got 3000 fps with 180's and 2850 with 200's, both of which are impossible in a .30-06.)
Just about any modern bottleneck cartridge gains something from a longer barrel and loses velocity in a shorter barrel. You gain quite a bit with a 26" barrel on a .270, 7mm-08 or .30-06 as well as the "magnums."
Muxxle flash is caused by hot gases reigniting when they hit the atmosphere, but unburned powder. At least 99% of the powder is burned in the first few inches of any centerfire rifle barrel.
