I was looking at the new Berger Web side.... why do they recommend
a 1-13 twist for the 68 grain bullet????
We have been shooting 68 grain pills out of a 1 -14 twist
barrels for 30 years +!
Magnus,
Go to the website and click on the Products link and then click on Target Bullets. In the 6mm section you will find the 6mm 68 gr listed (which you already know). In that listing is a bullet's length. Write that down on a piece of paper (or remember that it is .845). In the upper right hand corner of the products page is a link to a twist rate calculator.
Click on the link and you will go to an interface that will allow you to input all the details needed to calculate bullet stability. If you do this for the 68 gr bullet at 3,200 fps, 59 degrees and at sea level in a 1:14" twist you will see that the stability factor of this bullet under these conditions is 0.98. This is regarded as a bullet that will not be stable.
The reason BR shooters get away with using 1:14" twist is because the line is influenced by many factors. Most BR shooting is not done at sea level. Change the elevation by 1,000 feet. You will see that the stability factor goes to 1.02 which is on the edge but regarded as likely stable. Add to this that many BR shooters push the bullet faster than 3,200 fps. If your velocity is faster or if you are shooting in warmer temperatures the bullet's stability will be improved.
Since we have to provide a recommendation based on the notion that some shooters might use this bullet at sea level in very cold temperatures we go with the twist that gets us at or above 1.00 stability factor under these conditions (which are as hard on stability as it gets). If they have 1:14" twist based on our recommendation the bullets won't be stable and guess who gets the phone call.
Jackets and bullet tooling are very good these days as are the skills of several bullet makers. Spinning the bullet slow was adopted so long ago because slower RPM reduces the negative effects of imbalance. It is my opinion that many bullets made today are more balanced than bullet have been throughout history. Using a close to instability twist rate isn't needed as much today as it was years ago.
I know each shooter will decide for themselves what works and for goodness sake if it ain't broke don't fix it. However, if you end up at a match with a low altitude and it happens to be a cold day you may find your loads need to be really hot to start shooting tighter. This result can be related to twist rate. Go back to the twist rate calculator on our website and see what happens when you start over with all the same original data and change only the twist rate to 1:13" or even 1:13.5" and you'll see why we recommend a 1:13" twist.
Your question does draw to my attention something that needs to be cleared up. The 6mm BR Column bullet lists a 1:14" twist but it currently shows a .863 OAL which is wrong. It is more like .808. This makes a big difference in stability factor and it's the wrong dimension. I'll get them to correct it on Monday.
Regards,
Eric
