400 yards will be no trouble for a 308 with a 16-inch barrel, especially if you are concerned only with standard service rifle-level precision. When you want greater long range precision you can get yourself a rifle with longer barrel and a stock that's more amenable to it than the paratrooper stock. Your chosen rifle type determines precision more than a couple inches of barrel length.
As a general point, a longer barrel will give greater bullet speed, depending on choice of ammunition. For a given rifling rate, greater bullet speed will give faster spin and, for a given bullet type, it will give greater gyroscopic stability. Bullet spin rate does indeed depend on bullet speed.