Miller stability number

[powdergun]

I have been playing around with the JBM calculator on the stability of bullets. The notes say that a stability of between 1.3 and 2.0 is optimal and that military loads try to achieve 1.5. Some of my combinations indicate a stability of 2.5.

Is this better than the suggested range or should a load produce a stability in the range suggested

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

 

[AlexF]

I have been playing around with the JBM calculator on the stability of bullets. The notes say that a stability of between 1.3 and 2.0 is optimal and that military loads try to achieve 1.5. Some of my combinations indicate a stability of 2.5.

Is this better than the suggested range or should a load produce a stability in the range suggested

http://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi

Match quality bullets will shoot very accurately with a stability factor of 2.3-2.4.
For the same stability factor, standard millitary FMJs often shoot wildly because they are not well balanced. (a 55 FMJ M193 in a 1-7" barrel twist).
Optimal accuracy is always a around 1.3 - 1.4 for a given set of conditions.
A consequence of this is that a tight 1-8" barrel twist will only shoot 80gr to 105gr 6mm bullets very accurately while a 1-9" barrel twist will only shoot 70gr to 95gr bullets very accurately.

A simple way to think about this is that a spinning bullet behaves in many way like a spinning car tire: a well balance tire is stable at any reasonable speed while an unbalance tire woobles and jumps wildly at higher speeds.

Alex

 

[powdergun]

I'm looking for a good combo for a 7-08 with a 1 in 9.5 twist. Using 140 gr bullets I can't seem to find a reasonable velocity that will produce a stability factor in the range you suggest. However, researching loads 120-140g bullets are suggested as ideal for this set up.

I am confused

 

[Andy]

Your 1:9.5 twist will stabilize any 160 gr and lighter bullet made, at any velocity. I suggest you choose a bullet you think will satisfy your needs, and start developing an accurate load.

You'll be out shooting, while others will still be debating what should theoretically be accurate or inaccurate.

 

[ATOM]

It is possible to have too much stabilization, which is what I think you're asking.

Picture the trajectory of a bullet - a modified parabola. The changes are in the main due to drag - air resistance. Let's keep it simple and just say 'parabola'. Draw in your mind's eye that parabolic path from the muzzle to the target.

Understablized projectiles will of course start to wobble (often when slowing to around transsonic speeds, but in cases of gross understabilization, almost immediately). Accuracy will of course be the pits.

A bullet spun at the proper rate ('proper' of course depending on air density, calibre, bullet specs and so forth), will have its axis on that parabola at any given time. In other words, both bullet tip and the centre of the bullet's base are 'pierced' by the parabola. Such a bullet will land nose-on.

Over-stabilization - It is possible to spin a gyroscopically-stablized projectile so fast that it will maintain the same attitude throughout its flight, impacting the target at the same angle relative to the ground as when it left the muzzle.

OK, you're not going to see that very often with small arms. Don Miller's How Good are Simple Rules for Estimating Rifling Twist? (http://www.jbmballistics.com/ballistics/bibliography/articles/miller_stability_2.pdf) touches on this but correctly notes:

However, stability factors even as high as 3.5 to 4.0 are usually not detrimental to small arms shooting, which is at low angles of elevation ("flat fire").

(The article is pretty technical, but the first page is well worth reading if you're interested in stability.)

Miller notes that problems with higher spin rates (giving higher stability factor) are most noticeable with poor-quality bullets, making AlexF's comments about well-balanced and poorly-balanced tires a very good analogy for real-world small arms accuracy.

Bottom line? Go with Andy's advice:

I suggest you choose a bullet you think will satisfy your needs, and start developing an accurate load. You'll be out shooting, while others will still be debating what should theoretically be accurate or inaccurate.

 

[powdergun]

Thanks for the explanation. I have found it very useful.

All I need is a bit of good weather to get out there shooting.

Thanks again

Norm

P.S. perhaps I should start another premium vs fast expanding bullet as it relates to the Bang flop effect debate . Or not...

 

[AlexF]

Here are some values of S for different loads under normal conditions:

The loads the I shoot:
223 Rem 1-9
Nosler Custom Competition 5.56mm 68gr 0.90" 3100fps S = 1.69 1/2 MOA

243 Win 1-9.125
Nosler Ballistic Tip 6mm 80gr 0.99" 3100fps S = 1.72 1/2 - 3/4 MOA
Nolser Ballistic Tip 6mm 95gr 1.15" 3000fps S = 1.30 1/2 MOA

270 WSM 1-10
Nolser Accubond .270" 140gr 1.31" 3100fps S = 1.42 1 - 1 1/2 MOA

Some load that you could use for 7mm-08
7mm-08 1-9.5
Nosler Ballistic Tip 7mm 120gr 1.13" 3000fps S = 2.14 (at the limit but Nolser bullets are almost match quality)
Nosler Accubond 7mm 140gr 1.29" 2800fps S = 1.67
Nosler Accubond 7mm 160gr 1.41" 2700fps S = 1.45

From the numbers, I would be confident that both Nolser Accubond 7mm 140gr and 160gr bullets should be great choice and that even Nolser Ballistic Tip 7mm 120gr should be accurate.

Alex

 

[Andy]

Another thing to add:

- generally a bullet that is stable at 100 yds will remain stable, as horizontal velocity drops much more quickly than rotational velocity (rpm's).