questions about bullet weight vs twist ratio

[joejo]

apparently everything i have learned about twist rate vs bullet weight/ legnth is not all that accurate. i have been reading applied ballistics and have been looking over the .264 bullet data within the book. it seems as though a 123 scenar out of an 8 twist barrel is not nearly as stable as its bigger brother the 139. how can this be? i have always been under the assumption that if you went to a longer heavier bullet you would become less stable in the same twist barrel. it also shows poor stability with the cauterucio bullets in 8 twist barrels, even though they only range in the 119 to 130 gr range. can someone explain this? should i go with a 123 smk rather than a 123 lapua? this is the weioght range i want to be in. apparently the 123 smk is almost 1.5 times as stable in an 8 twist over the 123 lapua.
this trend is also consistant with the berger line. the 140 is apparently more stable than a 130?

 

[Greyfox]

You are right, this does go against convention wisdom, but then again the author applies fairly sophisticated formulas that take into account a lot more than just length and bullet weight. Size, velocity, shape, air density, etc. I think the practical aspect to the charts shown in the book is that if the twist rate exhibits a stability factor of at least 1.4, it is likely to be stabile. From the data shown, some bullet designs must be better than others irrespective of weight and length.

 

[LouBoyd]

Stability is also dependent on velocity and air density. The only reasonable way to determine if a given bullet will work in your rifle with your loads is to shoot in the highest air density conditions (coldest and lowest elevation) you expect to encounter.

I live and shoot at a mile elevation in Arizona. Published twist recommendations are are useless. You can try calculating stability with one of the many published methods including McGyro, but shooting tests are more trustworthy. If I took my rifles to a cold place near sea level they would shoot very poorly. In that unlikely event I'd have to change bullets and loads.

 

[Brimm]

weight has only a little to do with stability....contact with the bullet to barrel ratio is where it is at. Best program for this is the Sierra Infinity V6 program by Sierra bullets, was written by two aerospace guys, will give you the minimum twist rate you need for any bullet(probaby why your loads are unstable)
Brim

 

[Brimm]

the longer the bullet, the more contact with the barrel , the more need for faster twist. Hard to over stabilize a short bullet....easy to understabilize a long bullet.
Brim

 

[joejo]

the longer the bullet, the more contact with the barrel , the more need for faster twist. Hard to over stabilize a short bullet....easy to understabilize a long bullet.
Brim

then why do you suppose a 139 scenar with a bearing surface of .456 has a worst case scenario stability factor of 1.49 in an 8 twist but the 123 with a shorter bearing surface of .357 has a worst case scenario of .97 in an 8 twist?

 

[Brimm]

as LouBoyd stated....published data is just part of the over all scheme of everything. We can run numbers till we are blue in the face(too many factors) but when it comes down to brass tacks......we shoot....and we weigh the results with the evidence we gain.
Brimm

 

[Bullet bumper]

The length of contact in the barrel affects yaw of the bullet so that can affect the BC and stability. However length and the bullet design has more to do with the need for a certain twist rate . The centre of pressure and mass of a bullet is changed with the change from a flat base to a boat tail or the change from a low calibre ogive to a high calibre ogive etc . In every case the bullet mass is moved about with respect to the centre of gravity along the entire length of the jacket . A typical example is Barnes Varmint grenades .
the short bullets need a twist rate that is way faster than other heavier bullets the same length. Why ? Because the whole ogive is hollow and all the mass is in the base of the bullet moving the centre of pressure way to the rear and increasing the potential for yaw and to tumble or wobble and become unstable .
To understand why one bullet needs one twist and another similar profile bullet needs a different twist you need to understand and compute many variables .