Twist Rate Formula's

[Tman2ks]

I'm new here and am getting ready to build my first Grendel upper to use on my existing AR-15 rifle. Signed up a while ago and been reading and learning all I can. I have read the 5 pages that come up when using the search feature for twist rate or barrel twist. There are lots of posts by members saying this twist works for them and that twist works for someone else for different barrel lengths. What I didn't find was a formula or any kind of data as to why this rate was better than that. I did more internet searching and found these three links that actually have a formula and spreadsheet for twist rate. They allow you to change bullet weight, velocity, temperature and altitude to see what effect it has on which twist rate is optimal. They say optimal is between a gyroscopic stability factor (SG) of 1.5 and 2. I have no way of knowing the accuracy of them but after reading about them they seem good to me and match up pretty close to what has been posted here. Just thought I would share them. They do not use barrel length but velocity which would vary with barrel length. From other posters here, these are the appox. average velocity for different barrel lengths and bullet lengths.
16" 2420
18" 2470
20" 2510
22" 2555
24" 2595

BULLET LENGTHS
100 gr. Sierra H.P. .937"
100 Hornady SP .981"
95 gr. Horn. V-max 1.015"
100 AMAX 1.046"
120 Speer SP 1.061"
100 gr. Nosler B.T. 1.087"
120 gr. Sierra MK 1.187"
120 gr. Horn. A-max 1.214"
120 gr. Nos. B-Tip 1.221"
123 gr. Horn. A-max 1.240"
140 Hornady SP 1.252"
107 gr. Sierra MK 1.269"
123 SMK 1.301"
123 gr. Lap. Scenar 1.303"
129 Hornady SST 1.319"
129 gr. Horn. SST 1.327"
139 Lap. Silv. Scenar 1.358"
140 gr. Horn. A-max 1.372"
142 gr. Sierra MK 1.377"
140 gr. Berger VLD 1.422"

http://bulletin.accurateshooter.com/2013/02/find-optimal-barrel-twist-rate-with-berger-stability-calculator/

http://bulletin.accurateshooter.com/2008/06/calculating-bullet-rpm-spin-rates-and-stability/

http://kwk.us/twist.html

Found this technical article from June 2009 on the various stability factors for twist rates.

http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_2.pdf

One more calculator

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

[kmon]

http://www.bergerbullets.com/twist-rate-calculator/

Another that uses the Miller Rule

[Drillboss]

Tman, here's one purely empirical data point for your consideration. My barrel is a 20" 1 in 9 twist. I loaded up some 140 gr Berger target bullets today. I know 140 gr is outside of the Grendel's boilerhouse, but they were available and I like Bergers.

The first 3 groups were kind of all over the page (up to 6") and keyholed most of the hits. My final group was Berger's max load. At this higher velocity, the holes were perfectly round and it put two in the same hole with the 3rd about 1.25" away. Berger's manual (and the link above) recommends 1 in 8 twist for those bullets, but I wanted to try them anyway.

[nuthead]

If you're shooting long range (to me 600 to 1000m is long range in the grendel) i would always go for a minimally stabilized bullet if possible - an SG of 1.5 to 1.7 is good imo

Just looking at the numbers it would be hard to make first round hits much past 600m in any kind of a wind (basing this on a 123gr amax at 2600fps)

[LRRPF52]

One of the biggest surprises for me with this cartridge was shooting my 16" Grendel at 1200yds a few years ago. AA uses a 1/7.5" twist on the 16" pipes, so they stay spun really well.

If you run the numbers of a 123gr SMK from a .260 Remington vs. a 6.5 Grendel, you'll start to see why I shoot my Grendel's so much more. I see the real performance advantage of the .260 Rem at close ranges where you want the flatter trajectory, but once you pass a certain point, it's hard to see the difference between the two.

You're looking at 100-150yds difference between where the .260 hits the Grendel's muzzle velocity, given the same barrel lengths. I can buy boat loads of factory ammo for the Grendel for very affordable prices, whereas I don't even consider shooting any of the .260 Rem factory ammo in my .260 gasser.

I also like the Berger spin stability calculator. It's a great program, and has a built-in bullet list as well.