Not all rifles in the same caliber with a 1 in 12 twist will stabilize a longer heavier bullet. Maybe Berger bullets has developed a 64 gr. bullet that works in vanguards, but here's a simple explanation on twist rate.

Rifle Twist Rates

Rule of thumb : The heavier and longer a bullet is, the faster the rifling twist rate needs to be to stabilize it in flight. The lighter and shorter the bullet is, the slower the rifling twist rate needs to be to stabilize it in flight.

Expressed in terms of the number of revolutions per inch of barrel length, the ratio is commonly expressed by designations such as 1:10 or 1/10 or 1 in 10 twist. The 1 represents 1 twist, the 10 represents inches of barrel length. Therefore, a 1 in 10 twist represents 1 complete bullet revolution every 10 inches of barrel length traveled.

Higher numbers for this ratio indicate slower twist. Lower numbers for this ratio indicate faster twist.

Most rifle calibers have standard twist rates already designated, and will accommodate most of the bullets (weights/lengths) that are suitable for that caliber. However, rifle barrels can be made with faster or slower rates of twist ( special order ) within a particular caliber to satisfy the needs of specific shooters. (Swat Teams, Dangerous Game or Varmint Shooters) Assuming the rifle has a good crown, a crisp light trigger and the barrel is pressure bedded or floated, you should be OK using bullets that fall within the boundaries of the std. twist rate for your caliber and get good accuracy. However, all rifles have pet loads.

Using a general cal. choice such as the 223, here is an example of “not enough” and “too much” TWIST : If the rate of twist is insufficient to stabilize the bullet, (twist rate too slow 1 in 12 or 1 in 14 with heavy/longer bullets) it will cause the bullet to yaw and pitch by the time it reaches the target. If the rate of twist is too high, (twist rate to fast 1 in 9, 1 in 8, or 1 in 7 with light bullets) a high velocity bullet can begin deteriorating under the centrifugal force and possibly break apart before it reaches the target.