All this discussion about 7mm rifling twist rates makes me wonder... does a faster twist rifle shooting the same weight bullet at the same speed as a slower twist rifle give the bullet more stability IN game? or is there any difference in expansion / wound cavity / killing power?? Is it possible that's why the old style 6.5mm rifles seem to kill better than paper ballistics would indicate? or, What's your experience and theories?
It does matter at moment of impact. GS Custom bullet maker has written some notions about it. After the bullet deforms it retains stability with slower rotation.
If you think a spinning top. Narrower is harder to keep upright than wider. That’s remotely comparable to situation where bullet encounters the skin of the game. If the bullet was marginally stable in flight, at high velocity impact it destabilizes like spinning top at the end.
Now the resistance of air and living tissue are different. Air resistance is distributed load that applies pressure to whole frontal area and underpressure to rear of the bullet. In tissue, the forces concentrate on nose of the bullet. As result, in flight, faster is more stable. On impact faster is less stable.
As result, twist rate for ensuring stability at point of impact, until diameter expansion is sufficient is higher at close range. Impact velocity seems to be determining factor there. At longer distance it is necessary for the bullet to tilt along with trajectory so less stability is desired. Also with lower impact velocity the need for higher stability decreases. Now as the twist rate is fixed with the rifle, it’s necessary to consider the bullet chosen for the job.
Twist rates of factory barrels being designed for lead core bullets those have rarely any issues. The stability depends on the length, shape and diameter of the bullet, less on density as long as it’s reasonably uniform. As less dense material copper results in longer bullet, requiring higher twist rate. If hunter uses same 180gn bullet in 308 in copper as they used to do with old softpoint, the bullet is more than likely to destabilize on impact and hunter blames the bullet when they picked wrong tool for the job. Lead bullets used to be limited for velocity they can withstand for controlled expansion, for copper that isn’t an issue and in some calibers same goes for bonded core bullets.
Where bullet strength is solved, at short range, lighter bullet and higher velocity will always do more damage unless it’s question of marginal caliber where ability to cause large temporary cavity is sacrificed to ensure penetration.
