Found this somewhere else:
When you shoot a longer (heavier) bullet of a given caliber, you usually need a faster twist rate to stabilize the bullet. There is a point of diminishing return here. The faster you spin the bullet, the more noticeable flaws in bullet construction become. The kicker here is the bullet jacket. If there are minute wall thickness inconsistancies in the bullet jacket, one side of the bullet is actually heavier than the other. As the bullet spins, it starts to yaw, or another way of looking at it is the centrifugal forces start to pull the bullet off course. That's why the bench rest guys shoot light .22 and .243 bullets with slow rates of twist. They are shooting at 100 or 200 yards.
The 1 in 9 twist is marginal for the 142 gr Sierra Matchking, but is good for the 120 gr. stuff. Now, don't let this throw you. If you are a hunter, the 1 in 9 twist will probably be the best all around choice. It will stabilize the 140 gr. flat base, and shoot the 160 gr. round nose very well. It will also shoot the 120 gr. stuff well, without the chance of overstabilizing the bullet. Where you would need the 1 in 8 twist would be for the long VLD style bullets that need this much stabilizing. That's why the long range target shooters and 1000 yard bench rest guys go for the faster twist. It's a special application.
Also, don't let anyone tell you the slower twist will slow the bullet down. It is just a matter of proper application.
An example of this twist rate application is the M-16 used in Viet Nam. When the war started out in 1965 or so, (actually it was going on before that, but I think thats about the date the M-16 came in) the M-16 started out with 1 in 14, or 1 in 12 twist, depending on which report you read. There were all kinds of reports about the bullet "tumbling", and they were not far off. The bullet wasn't stabilized. When the army found the heavier bullet, 63 gr., penetrated body armor better at long range, the twist rate became 1 in 7 as it is found today.
