Barrel twist

[Methow Packer]

Does the bearing surface of a bullet dictate the rifle twist rate? Example a Berger 185VLD might have a .585 bearing surface and can handle say a 12" twist. A Berger 215 VLD with a .785 bearing surface needs a faster twist etc. Is there more to it than that? Ogive shape and boat tail size ? I have a Brux 12" twist barrel that may shoot 215 Bergers from a 300WM. The Hornady ELD-X 200 grain is maybe a little shorter than the Berger 215VLD. Could the Hornady ELD-X be a better fit for a 12" barrel?
'
Many thanks Ted

 

[MudRunner2005]

There's a lot more to it than just bearing surface, but yes, bearing surface is one of the many factors involved.

 

[MagnumManiac]

Bearing surface has little to do with it, VLD's have less bearing surface, generally, than normal style ogive bullets.
The most important factor of barrel twist is the total length of a bullet, the longer it is from base to tip, the faster it's spin needs to be to keep it stable.
Some bullet manufacturers have the 'ideal' twist rate on the box, such as Berger, but, that is not absolute, a slightly slower twist may still work, while a faster twist is rarely a problem, unless excessive for light/shorter bullets.
The amount of bullet that engages the rifling is moot, as long as it doesn't strip, it will follow the rifling down the bore.

Cheers.
gun)

 

[Lefty7mmstw]

hornady is listing 10" twist minimum on their newer boxes of the 200 eld-x... I have one box with the labeling and one box without...

As to what makes a bullet take a certain twist, it's the bullet's length and diameter....

 

[elkaholic]

Does the bearing surface of a bullet dictate the rifle twist rate? Example a Berger 185VLD might have a .585 bearing surface and can handle say a 12" twist. A Berger 215 VLD with a .785 bearing surface needs a faster twist etc. Is there more to it than that? Ogive shape and boat tail size ? I have a Brux 12" twist barrel that may shoot 215 Bergers from a 300WM. The Hornady ELD-X 200 grain is maybe a little shorter than the Berger 215VLD. Could the Hornady ELD-X be a better fit for a 12" barrel?
'
Many thanks Ted

Bearing surface, in and of itself, has little to do with twist needed. It is a primarily a function of length in relation to diameter but how the weight is distributed is also a key factor. Ex: a 215 Berger Hybrid is a long bullet that can get away with less twist than normal because approx. .4" of the nose cavity is hollow (no lead). Also, the elevation you are hunting (shooting) at affects the twist rate needed considerably......Rich

 

[Mikecr]

Putting bullet center of mass(Cg) further behind center of pressure(Cp) does not make it more stable.
But reduced drag can.

Better to remember what twist rate is; displacement per turn -to overcome that displacement.
So 8:1 twist rate is 8" of displacement by each turn.
Now displacement itself is relative to air density and resultant drag by design.
Therefore every aspect of a bullet's design that contributes to drag -affects stability.

Then you have disturbances to overcome(moments).
Gyroscopic turns are not the only way to overcome disturbances. Center of gravity -vs- center of pressure relationship is another. And this design attribute contributes toward or against stability.
This is the moment arm that makes longer bullets more of a challenge to stabilize, even while drag is reduced(because their Cg is usually further behind their Cm).
Another factor commonly overlooked is quality of bullet release. Too short of a barrel for the cartridge capacity leads to extreme muzzle pressures, which do nothing good for clean release of BT bullets.

 

[den]

Hornadys load data for the 308 win and the 200 gr eldx says they used a 12 twist for load data testing the box I have says 10 twist when I called them about using these in a 12 twist rum they told me the 212eldx @ 3100fps is 1.29 sg, & the 200 Eldx @ 3100fps 1.48 sg I beleive they said this was at a standard air temp and sea level but not a 100 % on that