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+--- Thread: 12" 6.5G Barrel Twist Rate? (/showthread.php?tid=18425)
12" 6.5G Barrel Twist Rate? - ibshane - 03-01-2019
Should a 12" barrel have the traditional 1:8" twist?
I see some of the shorter barrels out there have a 1:7.5" twist.
When I do get a 12" barrel, I plan to still shoot 120-123 gr. pills.
12" 6.5G Barrel Twist Rate? - BluntForceTrauma - 03-02-2019
Using Berger Bullet's twist rate stability calculator, a 12" bbl with a 1:8 twist launching Berger's 6.5mm 130 AR Hybrid bullet at 2000 fps at 0 altitude and in 0°F temperature is stable (think the coast of Alaska in winter).
Lighter bullets will also be fine, of course.
Using the same setup with Berger's 140 Hunter Elite at 1900 fps loses 2% BC in a 1:7.5 twist and 8% in a 1:8 twist barrel.
So, a 1:7.5 twist will lose 6% less BC if you're shooting 140-grain bullets in winter at sea level than a 1:8 twist.
12" 6.5G Barrel Twist Rate? - KentuckyBuddha - 03-02-2019
Wouldn't a 1/7 twist be better if one were say at 1000ft (304m) at 0F (-18C) even with 123gr?
12" 6.5G Barrel Twist Rate? - kmon - 03-02-2019
The 130gr Hybrid BFT used in his example is longer than the 123gr offerings that I know of and would require more twist to stabilize. Gaining altitude and/or temperature will increase stability of the bullet. The example he gave above gives a SG=1.50 give it 100 degrees and 6000 feet elevation and SG=1.81. At 0 degrees and 1000 feet elevation SG=1.56
Any SG 1.5 or over is considered stable and you are not loosing BC due to instability.
Here is the link for the Berger twistrate calculator, you can choose one of their bullets or enter your own data for the bullet you use or plan to use.
http://bergerbullets.com/twist-rate-calculator/
12" 6.5G Barrel Twist Rate? - BluntForceTrauma - 03-02-2019
My calculations gave sort of a worst-case scenario, under the assumption that most Grendel shooters won't be focusing on 140-grain bullets. So, any bullet lighter than 130 grains, and any conditions that are warmer than 0°F and any altitude higher than sea level performs better and better.
Bottom line: 1:8 is probably the best compromise twist rate for Grendel barrels from 12" to 24".
If one wanted to shoot only heavy bullets, I suppose 1:7 would be best for a 12" in extreme conditions and 1:9 might be best for a 24" (which is what Bill Alexander used to do before he left Alexander Arms).
I generally subscribe to the school of thought (and I get this from ballistician Brian Litz) that—all things considered—it's best to slightly "over-stabilize" a bullet, to have a slightly tighter twist than absolutely necessary, than to swing the other way.
12" 6.5G Barrel Twist Rate? - ibshane - 03-04-2019
Thanks!