Re: Berrel length?
The 308 is an excellent very efficient cartridge using faster burning powders in that weight bullet and does not need long barrels for shooting. Velocity gain with longer barrels in the 308 would be minimal but would increase some if you used the heaviest bullets with slower burning powders. Short and stiff is actually better with this cartridge. A guy I shot with a good bit in Colorado had a 24" heavy barrel that shot like a dream. He shot out to the 1000 yard targets and did very well with it.
Predictions are difficult, especially when they involve the future