8, 8.5, or 9 twist for 7mm

[ericskennard]

Has anyone got experience with the 1 in 8 or 1 in 8.5 twist barrel using the Berger 180 VLD bullets? I was wondering if these bullets would work better with the faster twist of the 8 or 8.5 vs the 1 in 9???

 

[spclark]

Berger recommends 1:9. They know what they're doing. Faster twist doesn't always mean better accuracy.

You can put the same question to Eric Stecker - he's on this forum at: http://www.accurateshooter.com/forum/index.php?action=profile;u=425628

- or contact Brian Litz for a definitive answer; he literally "wrote the book" on modern rifle ballistics. He can be contacted at Berger: bryan.litz@bergerbullets.com

There's a stability formula in the appendix of his book on ballistics; here's a good treatment of the subject:

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

- including a link near the bottom to an Excel file that will let you massage your numbers for yourself.

 

[rcw3]

I have shot multiple 8.4 and 9 twist 7mm barrels with the 180's and both shot very well and there were no indications of stability or accuracy issues. In my mind an 8" twist is a little more than necessary so I never purchased one with a twist that tight.

Robert Whitley

 

[camac]

I agree 100% with these guys but would like to add a little more for consideration. Sorry I don't have much actual experience in shooting the 8twist to give real feedback but will give you some more to think about. The point I discuss heavily with customers when building a rifle is torque. The 180's are a reasonably heavy bullet and can create a lot of torque. The more twist, the more toque. Not only does this become harder to deal with for the shooter but also the gear. Round actions have difficulties keeping solid confirmation with the stock, stocks have dfficulties tracking in the bags etc etc. Less recoil and less torque are definately desirable traits for shooting well. Not that you can't shoot them well and learn to deal with it but it does make a difference. 8 twist will have approximately 11% more torque than 9 twist and potentially less velocity.
In thinking about building a 7mm for long range work and 180's I would consider torque carefully and think about implications for not only twist but also actions (recommend large diameter or flat to get a good grip on stock), Long tenons, offset stocks etc etc.

 

[wnroscoe]

I run a 1-9 in my 7/300wsm and have had no issues with the 180's. About 400 rounds through it now. I've also built several rifles chambered in 7/300WSM, 7WSM and 7SAUM. All for the 180 Bergers and all have had very good success with same.

 

[ajax]

does temp play a factor in the bullets ability to stabilize? 180gr bullets shot at warm temps that shoot good, how about at colder temps? would a guy need alittle more twist?

 

[Yellow11]

Apply Don Miller's formula and the out come is 1-9 twist.

The formula is based on MV, bullet length and weight - no reference is made to temperature

 

[habu]

BERGER RECOMMENDS 1-9 TWT OR FASTER.

AM USING BRUX 1-8.5 AND TRUFLIGHT 1-8

BOTH SHOOT 7'S EQUALLY WELL.

HABU

 

[TonyR]

I'm going to shoot some B180VLDs out of a 1:9 at 1000 yards tomorrow. It's supposed to be 32 degrees F when we start so that will be very interesting.

 

[spclark]

That's the key Ajax, cold air is dense air & can be the one factor that puts you over the line of stability if you're already shooting the edge. The trick is knowing how much faster is safe! Too fast & pressure can peak while the bullets get spun up to speed, and boosts the incidence of flyers down range.