Re: Twist for 308 WIN???
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: AshesToAshes</div><div class="ubbcode-body">See Don Miller's stability formula here:
http://www.jbmballistics.com/cgi-bin/jbmstab-5.0.cgi
Ideally you want a value of 1.3 - 3.0. Below 1.3 suggests the projectile will not stabilize with the given twist/velocity. Above 3.0 suggests your accuracy might improve by using a slower twist. The 175gr MK has a stability value of 1.173 @ 2600 FPS in a 1x12" barrel, which is below marginal. It's 1.335 in a 1x11.25" barrel, which is acceptable. A 1x10" barrel yields 1.689 which is much better.
Similarly, the 168gr (which has a length of only 1.21" as opposed to the 175gr's 1.4") stabilizes just fine in a 1x12" barrel with a value of 1.717. But it will not stabilize it at subsonic velocities (need at least 11.25" for that).
I was a bit skeptical about this formula before but after using it for a while it seems to be fairly sound. It's much better than guessing. </div></div>
This will cause people to think that a higher value is better. That is not the case. I am of the opinion that one should use the slowest twist that will statically stabilize the bullet at the muzzle. One needs to understand why spin is required in the first place and then how this changes during the flight of the bullet.
The spin rate is a function of the barrel twist rate and muzzle velocity. The spin is required to counteract the overturning effect air resistance has on the bullet, because of the elongated shape of the bullet. The greater the air resistance the greater the overturning forces and thus the faster the required spin rate to act as gyroscopic device. Also, the longer for caliber the bullet is, the greater it propensity for overturning, so it needs a higher spin rate to stabilize. Because spin rate is a function of twist rate and muzzle velocity, you can sometimes make up the required spin rate by upping the velocity, but that also cause the required spin rate to increase, albeit more slowly.
The downside of a high spin rate is that it can induce secondary instability into the bullet if said bullet is not perfect. This will manifest itself with precession that in more extreme case may even cause the bullet to deviate from its course and open up the group. This is why benchresters use "perfect" bullets at the lowest spin rate needed to stabilize.
Once the bullet leaves the muzzle, if it is statically stable at the muzzle, it will be stable all the way, until it reaches either the target or transonic velocity. During the transonic passage, the bullet may become unstable, like the celebrated case of the 168gr SMK, but not always. Most bullets will remain stable. However, before the transonic range or the target, is attained, the bullet actually gains stability because the forward velocity decreases and the spin rate does not. As it approaches the transonic range, the pressure does start to build up again. This is why the bullet should be supersonic when it gets to the target and the faster the better.
All this to say that I would not simply get a 1:10 twist if I would never use bullets that required the spin rate generated by that twist. I would get a twist rate that will properly (but not overly) stabilize the heaviest (longest) bullet I am going to use.
