the twist of a particular gun is set to accommodate the (typically) longest bullet commonly available in that caliber. The best two examples are the 270/7MM Mag and ther 6MM rem/.243 win....

The diameter difference between a .270 and a 7mm Mag is only .007 inch. That' s only the thickness of two human hairs....yet the 7MM bullets commonly include the 175 grain spitzers as opposed the 150 grain bullets for the .270. The 7mm rifles all have (roughly) 1 in 9 twists to handle the longer bullet and most all .270 rifles have 1 in 10 twist. The faster (1 in 9) twist is required to stabilize the longer bullet.

This does not mean that you can' t shoot shorter bullets in the 1 in 9 twist barrels.....you sure can!!! In the 7mm you could shoot a 120 grain bullet with a 1 in 14 twist, but you' d not like the results trying to shoot the 175 grain bullets in that same barrel.

Typically I say that with hunting rifles .264 and larger I prefer the faster twists because I' m likely to be shooting the longer bullets. However with varming bullets in .224 and .243 diameter, I prefer the slower twists because I often prefer to shoot the shorter bullets and at extreme velocities and the faster twists can cause such extreme RPMs that the bullet actually explodes from centrifugal force in mid air.....the bullets also tend to be thin jacketed as well.