Originally Posted by Gardien
My only quesiton has to do with barrel twist. I have read nearly everwhere that a faster twist rate is required to stabilize heavier bullets, but I got from your article that the opposite is true (p.2 paragraphs 2 & 3).
Can you help clarify this?
Again, thank you!
You are correct in that heavier bullets require more twist. Technically it has more to do with the density of the materials used and its weight/length relationship. If we could use tungsten for bullets, a 200 grain 30 cal bullet would need less twist than a 200 grain lead bullet and a 200 grain lead bullet needs less twist than an all copper bullet.
Since we use mostly jacketed lead I will base any statements on jacketed lead.
Another factor that involves twist is bore and bullet diameter. The smaller the caliber and heavier the bullet for that caliber will need a tighter twist. For example a 175-180 grain 30 cal bullet really only needs 12x - 13x for proper stability where as a 180 grain 284 cal needs about a 9x for proper stability.
The larger the caliber, the slower the twist typically needs to be for average bullet weights. Granted there are 30 cal bullets that require a 9x but are very rare. Most 30 cal rifles will do their best with 11x or 12x depending on bullet weights. A 10x is needed for the 240 SMK. Most shooters would go with a 9x for a slower round like the 308. The 11x will handle up to the 210's even at 308 velocities. That is about more bullet than is practical for the 308. I use the 208 on occasion but more often than not I am running 155's through 200's with the 168-180 being shot the most. With that in mind I use a 11x for all of my 308 needs. If I were going to use a maximum of 190 grains I would use the 12x for my 308 needs. You can run the 200's in a 12x but my opinion is that 12x is very marginal for 200 grain pills especially the newer ones like the Accubonds. 12x and the 200 AB works ok for the 300 RUM's but not so much the 308 due to the lower velocity.
Having said all that, bullet weight for bullet weight and specifec gravity for specifec gravity, the 30 cal requires less twist than a smaller caliber. This slower twist allows for slightly better velocity/pressure relationships and less radial torque during recoil. The same rules apply when switching from the 30 to 338 or 338 to 375 calibers. The bigger the bore the less twist is required for equal construction and weight compared to a smaller caliber.
I hope that helps!