Hello Folks

Recently I asked about the probability of my 250 Savage stabilising 110 gn projectiles at 2700 fps with a 1:10 twist. I had already had a look at some of the online stability calculators (this is the one I like - http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi ) and as usual bullet lenth is the critical factor.

However with the interbonds or accubonds its easy to knock or cut the plastic tip off, and reduce the length considerably - e.g. on the 110gn 257's it reduced by 1/10th of an inch) - with a huge effect on "calculated" stability !!

Could/would reducing the length by the amount of the plastic tip make such a big difference to stability ? Can anyone explain the physics of what is happening here.

Thanks

The bearing surface is what makes a bullet short or long when talking about staiblizing and rate of twist. I don't think any .257" 110 grain bullet will stabilize with 1/10" twist barrel.

Tentman:

It is all about mass in motion and more so the spatial distribution of mass in a body with set dimension, shape and volume.

The change in length of the projectile in this case has more to do with a change in the location of the point of pressure in the projectile relative to the projectiles centre of gravity; and then as result a subsequent change in the magnitude of the ratio of the projectiles longitudinal moment of inertia vs the transverse moment of inertia.

In other words the difference is in the location of the point of pressure relative to the projectiles centre of gravity.

By shorting the distance between the CG and the PP , the leverarm of the overturning force ( overturning moment) that is induced by lift is lessened and thus the calculated stability number would be greater than when the leverarm is longer.

The stability number is nothing more than a ratio of the gyro moment relative to the overturning moment.

If the Gyro moment is big relative to the overturing force then the calculated SF is larger, if it is small then the calculated SF is small.

In essence you need a larger stabilizing gyroscopic moment to counter the effects of a long leverarm that is in effect in a long bullet.

If the bullet short you need less gyro force to keep it stable.

Thank you Alf - I follow your explanation and take from it the due to the very low mass, the plastic tip shouldn't really have much effect wether or not the twist will stablise the projectile.

I am going to carry out a range test to see if I can detect a difference with tip on vrs tip off.

The stabilizing effect of any rate of twist on any bullet is a complex interaction involving it's individual mass, specific gravity, length, velocity, and overall shape. You cannot tie the effect of twist solely to bullet weight alone.

I would shoot those bullets to see how they perform and I'd shoot them at 100, 200, and 300 yards...more if you can. If they aren't stabilized enough the longer distances will shot that. Watch the shape of the bullet hole in your target.

It's not so much the mass of the tip itself but the position of the point of pressure that's at issue. The point of pressure is then also a theoretical point in that one cannot , unlike the centre of gravity actually measure where the PP is located.

For purposes of calculation of the stability factor the length of bullet from tip to CG is taken. That would account for the difference between the actual length of the leverarm vs the assumed length based on the above.

The CG of a bullet or canon projectile can be measured on a balance so can the Axial and Transverse moments of inertia.

There is a small difference between the calculated values for SF and the actual measured values as measured in a wind tunnel using scale models of a projectile.

I do not know about complex or difficult in terms of interaction between twist and stability?

The Molitz stability equation is valid and easy to apply if the true bullet data is available. By true I refer to data not usually supplied by commercial bullet manufacturers. Sierra and other selected manufacturers have the data available at request on certain bullets.

Years ago RWS and DWM made bulletins available on request on the ballistic data of their bullets.

Since Molitz published his book in 1963 on the mathematical calculations in exterior ballistics ( Aussere Ballistik 1963 Springer Verlag) things have gotten much easier to understand.

Molitz's formulas form the basis of many applications in aeroballistics including calculations used in space flight.

Nosler 25 caliber 110 grain Accubonds perform very well out of my daughter's Ruger Hawkeye in 257 Roberts, with a 1:10 twist.

Tentman, I had a real brain fart! I was thinking of a 1/10" 243/6mm with a 110 grain bullet. Of course your rifle should stabilize any standard cup and core or bonded 110 grain bullet. Sometimes my mind is totally fickle.....sorry.

You sure it is a 1:10 twist? I believe the early .250-3000 Savages had a 1:12 (1:14?)twist. In any event it would probably be easier to try some than to worry it to death.
C.G.B.

Yep - its 1:10 -its a brand new Kreiger on a Model B sporter K action.

It did stabilise the 110's ok (tested at 100 and 200 yards) but has other issues with bedding which prevented me from getting it to really group. More work required !!

Thanks for your ideas guys.