Twist rate and bullet selection

W

Wendyj

New member
308 is 1-11 twist. Been shooting 165-168 gr. would like to try 120 and 170 -180. Is it worth it or is twist not correct?

7 mm mag 1-9.25 twist. Best groups 140 nbt. IMR 4831 powder. Loaded some 150 sgk. Boat tail for the weekend. Thinking of some heavier bullets. How heavy grain will it shoot in this twist. Saw a box of Hornady 168-170 grain on sale at Acadamy sports. Don't know if 9.25 will shoot them or not. Accurate that is?
 
And here's another calculator that does it by bullet data with more detail. This will show you what is optimal.

The calculator above is based on Robert L. McCoy's old McGyro program. The JBM site uses a simplified estimate worked out by one of McCoy's student and coworker, Don Miller. Neither is dead on every time, but usually agree pretty well.

The most basic difference is the Miller estimator give you gyroscopic stability factor, while the McGyro-based calculator assumes you want a twist that gives you a gyroscopic stability factor of 1.5, and gives you a graph of what that twist is, verses different muzzle velocities.

Sierra told me long ago that they recommend a gyroscopic stability factor or 1.4 to 1.7 for maximum match accuracy, and 1.3 to 3.0 for "hunting accuracy". 1.5 is probably the most commonly cited as "optimal", though Harold Vaughn liked 1.4 better, and one authority (I am failing to recall which one) liked 1.7 best. But frankly, I've seen darn good accuracy from Sierra .308" 168 grain MatchKings all the way up to about 2.5. The common service rifle 10" twist, originally intended for 220 grain round nose .30-40 Krag ammunition, is too fast to be optimum with any of the more common 150, to 180 grain weights, and yet people swear even stubbier 125 grain bullets shoot well from them. This is why the SAAMI standard barrel in .308 Winchester has a 12" twist. The 11" twist was popularized by, I think, the Navy Marksmanship Unit originally, and that would be for the long 173 grain M1 Type match FMJ bullet used in M118SB.

The reason to avoid spinning too fast is every little bit of mass asymmetry starts to contribute wobble to the bullet's flight, plus you can cause core stripping, where the core slips inside the jacket, if you apply too much rotational acceleration. Either opens groups up. Too slow a spin opens them up, too because the bullet can't settle yaw out quickly enough to stay as cleanly on path. I take that 1.3 lower limit seriously. I've never had a bullet turning at below 1.3 that shot a tight group.
 
Guys, I really appreciate that info. If I just google the twist and bullets everyone argues over what theirs will shoot. I don't mind buying 50 bullets to give them a try. Bought some 165 accubonds for it and it hated them. 308 that is. But would drill 165 and 168 Nosler bt's all day. Accubond seemed built quite similar. Powder didn't variate a lot from Noslers. Laddered 2 powders in about ten loads. 2 inch groups at best at 100. Noslers seemed to stay in lr 15 and IMR 3031 around .7. Give or take.
 
Could be some other factor than stability. The Accubonds are probably a little harder, and that can affect barrel time. Could have been a bad batch, too. Occasionally that sort of stuff happens.
 
The shape of the bullet and the bearing surface-as well as where the bearing surface is located can have a lot to do with how well the bullet works in your particular barrel. The Leade also can have a lot to do with what is accurate.

My 308 Winchester is a Remington 700 Varmint Synthetic with a 26 inch heavy barrel. 1:12 twist. Typical long Remington style leade.

It stabilizes most everything from 150 to 180 grain weight bullets except for the VLD styles. I just cannot get the bullet close enough to the rifling to make it accurate and still feed from the magazine.

Generally speaking, Tangent ogives are more forgiving, or more self aligning than Secant ogives.

Some interesting reading:http://www.accurateshooter.com/ballistics/tangent-vs-secant-vs-hybrid-ogive-bullets/
 
I have used a variety of 180s in a 308 with 1:11 twist with great results as long as it was a grain or so within max charge. At 2 grains less than max, it patterned like a shotgun (okay, that a bit of a stretch). I check my.loads at 100 yards and 200 yards and with loads that do well, stretch it to 300 to confirm.
 
The Berger Stability calculator
http://www.bergerbullets.com/twist-rate-calculator/

My best groups [ 0.2 moa and 0.3 moa] were shot with 65 gr Vmax in 6mmBR chrono between 3122 fps and 3159 fps. with 14" twist that calculated as 0.97 stability

Some of the 75 gr Vmax from that barrel are VERY accurate, and some are keyholing.

What does it all mean?
The closer to unstable the better.
 
Anything from 125's up to 200+ gr will work with a 1:11 twist in a 308. The 125's might be borderline, but they'll work. The heavier the better.
 
Didn't have much time so loaded 5 with 40 grains and &&5 with 40.7. Will see tomorrow. First time with this powder. Loaded 10 168 gr with some too. Had some Sierra game King 150 gr loaded with reloader 15 and some IMR 3031. Laddered up from 39 gr to 41.5. Both powders. Didn't get to shoot last weekend so anxious to go.
 
You must log in or register to reply here.
Back
Top