12-30-2015, 04:19 AM
BluntForceTrauma Wrote:I read on another thread here on the forum that the twist rate on the prototypes is 1:9. I strongly recommend 1:8 for the 6.5 Grendel.
If you go to Berger Bullet's Twist Rate Calculator and enter in data for a sort of worst-case scenario 6.5mm 130 Berger AR Hybrid at 2450 fps at temperature 0 and altitude 0 with a 1:8 twist you get a "comfortable" stability factor of 1.61. Good article on this by Litz here.
Changing only the twist rate to 1:9, you get "marginal" stability of 1.27.
To get back to comfortable stability with a 1:9 in those conditions let's say you're on the Alaskan coast in winter you'd need a muzzle velocity of 4,000 fps!
Howa: Please, change the twist on your 6.5 Grendel mini-actions from 1:9 to 1:8 before these go into production!
P.S. If you're trying to be efficient by using the same barrels as your 6.5x55s, they, too, will benefit going to 1:8.
P.P.S. Bryan Litz tested velocity loss from twist rate increase and found it averaged only 1.33 fps per inch of twist rate. So if your 1:9 shoots 2601.33, then your 1:8 will shoot 2600 fps.
Sounds good. Well pass this along.
From my personal experience... And I'll upload pictures tomorrow... I shot nothing but sub 1" three shot groups at 100yards with factory Hornady superformance sst 123gr rounds. Wind was around 10-12mph. I managed to keep the 20" HB to .800" center to center and the 22" std bbl to .720" with the 1-9 twist.