Good question!
I have several Contender barrels, all of which are 10 inch. Perfect length, as far as I'm concerned. It fits in my day pack real nice.
I recently aquired a 15" Encore in 308. I had pretty much made up my mind to cut it to 12 inches but have since decided I will shoot it first. If it handles OK, I will leave it as is.
I too would like to hear from anyone that has a 308 in shorter than a 15 inch barrel.