Now which model rifle are you talking about.
The reason for the different twist rate is probably due to the length of the barrel used and the projectile it is designed to propel.
Length, weight and shape of projectile determines which twist rate might work best. Using the Greenhill formula - the equasion for a bullet traveling at about 2800 fps - which is a fast round for the 30-06 - the result is a twist rate of about 10.9 for a 150 gr. / 30 caliber bullet.
Since the SAKO engineers knows this - it is probably reasonable for us to assume that this is going to be about the fastest that a 30-06 bullet is going to travel down their barrel - so it is probably the determining factor as to where they set their twist rate for their barrels.
The reason why I use 30-06 is because the 30-06 is the round by which all other rounds are measured.
Twist rates are determined by the manufacturer for the speed and size of the projectile that comes out of the barrel. Hence a faster bullet would need more stability then a slower moving round.
The heavier the bullet the slower it usually travels through a 30-06 type gun and the less gyroscopic it is - due to the fact that the bullet tends to be rear heavy and falls out of suspension easier and is more susceptible to yaw and tumble.
One other fact is that the slower twist rate barrel lasts longer then the faster twist rate barrel - due to the fact that barrel will wear faster with a faster twist rate - throat erosion occurs faster and when the barrel wears out - the accuracy will suffer.
That is the reason why the .220 Swift barrels never lasted very long compared to a .30-06...
