help on rifling twist

[lboom]

I necked down a 7mm dakota to 6.5 mm and am getting ready to build up a gun for it, What rifling twist shoul I use if the Max grain bullet is 120. All the formulas talk about bullet length but is it overall length or land bearing length. I would like to be able to shoot 95 grain vmax to 120 grain sierra matchkings. Thanks Larry

 

[outofayr]

A 9 twist will easily stabilize 120s, and some/most 140s. Longer 140s like the AMAX and 160s will benefit from a faster twist. I'm shooting the AMAX through a 8.5 twist with no problems.

Good luck with your build!

Brian

 

[Fiftydriver]

I have never had a problem stabilizing any bullet up to 140 gr in a 1-9 twist barrel chambered in my 6.5mm Allen Xpress which is based on the 300 Dakota parent case. Slightly larger but still in the same performance class.

I would probably still recommend the 1-9 twist just so you were not limited to the 120 gr bullets. Who knows there may come a situation where a 140 gr bullet would be a better choice. If you have a 1-9, your set, if you have a 1-10, stuck with the 120s for the most part.

The velocity difference between a 9 and 10 twist would not be worth trying to figure, extremely low if any.

 

[lboom]

If I am shooting the 95 grain bullets 3900 to 4000 are they going to stay together with the fast twists and be accurate? I have heard that to fast of twist will cause the bullet to explode at the high velocities due to centrifical force????

 

[Ernie]

First off. I would stay with the recommended 1-9 twist. There are a number of reasons why bullets fail: bullet construction, rough or smooth throat/rifling, speed, and likely an overly extreme twist for the bullet(s) used, and the barrel dimensions itself.
Probably the bullets with heavier jackets will hold up better for what you are doing.
SMK' usually do weel at high velocity and the heavy jacket Berger's would be a possibility as well (maybe others but those two come to mind first).
E