Stability: Fine Points to be Aware Of

[nralifer]

The Miller Sg estimator:

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The Berger Stability Calculator:

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Altitude & Barometric Pressure:

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Running my home-made spreadsheet using the Miller method & comparing the Sg estimates with the Berger stability analysis for the Berger 85.5 .224 bullet, 7.7 twist, 3050 fps, 89 deg, 0 sea level, I produced a Sg of 1.43 with my home-made number fixer & the Berger Sg analyst stuff produced a Sg of 1.42. Berger than stated the Sg of 1.42 was "marginal".

Going into this the gyroscopic effect:

"Gyroscopic motion is the tendency of a rotating object to maintain the orientation of its rotation. A rotating object possesses angular momentum and this momentum must be conserved. The object will resist any change in its axis of rotation, as a change in orientation will result in a change in angular momentum."

In addition - precession - coning motion:

"Torque-induced precession (gyroscopic precession) is the phenomenon in which the axis of a spinning object (e.g., a gyroscope) describes a cone in space when an external torque is applied to it. The phenomenon is commonly seen in a spinning toy top, but all rotating objects can undergo precession."

Like move 90 degrees from direction force applied then rotating point describes a cone.

The Miller Sg estimator process applied by my home-made Sg number fixer & the Berger Sg analyzer accepts data like twist, velocity, pressure, temp. Looking at velocity & twist: a ratio of diameter H131/ twist H135 and a comparison of velocity H136 - 2800 (a standard) is used.

The rotational stuff like RPM's would be regarded as constant over the bullet's relatively short TOF. Per Berger, factors like twist, velocity, & air density would affect stability, like lower Sg values and less than optimum performance, less than the optimum BC.

I like the Hornady 4DOF ballistic calculator, it shows Sg's increasing down range. Initial Sg's are affected by MV & twist rate. Features like aerodynamic jump & spin drift are included. "The barrel twist, velocity, and air density will determine the muzzle Sg along with other projectile properties included in the projectile file."

The spin of the bullet does change,it decreases, albeit at a slower rate than the linear velocity. What doesn't change is the length of the bullet, a major factor in determining stability.

 

[nralifer]

An expanding bullet traveling through tissue gets shorter which tends to increase stability. The trick is to get immediate expansion by a nose forward attitude at impact. Certainly other factors come into play which can affect in- target stability and trajectory of the bullet. Asymmetric expansion will steer the bullet off a straight path. That is why excessive precession before impact needs to be avoided to assure as straight an impact as possible to get that initial symmetric expansion.

 

[orkan]

I can confirm, with absolute certainty... that if the twist rate and bullet remains the same, and you increase velocity, you gain more stability. I have seen this many times with many bullets throughout my career.

What is not stable in a .223rem, can be made stable in a 22 creedmoor of equal twist. So, RPM most definitely has a bearing on the stability of a projectile. That explanation might be an oversimplification from a ballistic terminology standpoint regarding what is actually happening, but the fact that it happens is not up for discussion.



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[nralifer]

I can confirm, with absolute certainty... that if the twist rate and bullet remains the same, and you increase velocity, you gain more stability. I have seen this many times with many bullets throughout my career.

What is not stable in a .223rem, can be made stable in a 22 creedmoor of equal twist. So, RPM most definitely has a bearing on the stability of a projectile. That explanation might be an oversimplification from a ballistic terminology standpoint regarding what is actually happening, but the fact that it happens is not up for discussion.



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I agree. The RPM calculation is straight forward, and it's the spin imparted to the bullet that stabilizes it. To get adequate stability you have to spin it sufficiently.

 

[QuietTexan]

There is a difference between gyroscopic and dynamic stability, they're highly interrelated but not the same thing. The effect Greg is seeing is real (higher velocity = longer bullets more stable at a given twist rate) and has to do with dynamic stability and how the bullet responds to external inputs. A 22-250 shooting a Berger 85.5 (1.170" long 85.5gn) bullet at 3100 FPS in an 7 twist will obviously have a higher bullet RPM and higher Sg factor than a 223 Rem shooting the same bullet in the same 7 twist at 2600 FPS. That bullet will be acceptably gyroscopically stabilized when it exits the muzzle at each velocity (1.64 Sg vs 1.55 Sg), but the dynamic stability Sd factor tries to describe what happens after that point. The bullet has forces imparted upon it while flying, if the Sd is low those inputs will have more of an impact. A faster bullet (to a point) should be more resistant to external effects, and will have a resultant higher Sd. The 223 Rem shouldn't be having those 85.5gn bullets keyholing at 100 yards, but the 22-250 should also outrun it at every range.

The reason the 168 SMK suffers from transonic instability has to do with how in that particular bullet design there is a cliff in the Sd calc where the transonic velocity reduction has significantly more impact on stability than the same absolute decrease in velocity in the supersonic range. Robert McCoy wrote a book that shows the math behind Sd, it was enough for me to not worry about it and generally go with "faster is better unless the target sucks" as a mantra

Pretty much every stability issue can be solved by matching components, if a bullet is unstable in a bore either change the bullet, change the twist, or change the cartridge. No one ever sits around and goes "this is fine" when something key holes Tends to be easier to find a lighter bullet than get a new barrel though.

 

[cape cove]

Great discussion.

 

[Mikecr]

I threw together an example of stability -vs- velocities, with a marginal twist rate.
It shows that you cannot rely on more velocity, or more RPMs, to bail yourself out of a poor twist rate.
And I went ahead and threw in [by 500yd] numbers.

Again, no amount of RPMs could bring this bullet to fully stable at the muzzle.
But thinner air would.

 

[orkan]

Define "stable at the muzzle."

Mathematically?… or practically?

I've had phenomenal performance from bullets and twist rates that the "math" says will not work… yet the combo is a laser.

I feel the math and charts often leave a lot to be desired when weighed against first hand experience.

 

[Mikecr]

Well my math is borrowed from that developed at Aberdeen proving grounds from direct measure.
It is published and proven.

RPMs were likely proven to mean nothing before we were born.
So today stability requirements do not include RPMs.
Have you noticed?

 

[orkan]

I'm not arguing previous works. I could happily sit here and talk complex ballistic theory and go through all the math too, and I have in the past.

I'm saying, there's more to this. The difference between practical application and academic discussion. Which is where so many of these threads come off the rails. Being right mathematically is only ever so valuable for a shooter. Shooters are not mathematicians. What works, works… and what doesn't work, doesn't… no matter what the numbers claim. It's always been that way in this discipline. That tells me that the math needs to be improved.

For instance, what does the math say about a 55gr .20cal Berger? Berger says 1:8 twist minimum.

Yet I have several .20cals that are running 1:9 and its unbelievable how well they shoot, both up close and far away.

 

[Hugnot]

TOF 1 second for a long-range shot. Bullet RPM 250,000 - how much will the RPM decrease? If so, how much & what will cause the spin decrease. Bullets have smooth sides except for rifling engraving that provides negligible rotational air resistance, TOF is short, RPM's will only slightly decrease during TOF - about 5%. This assumes the bullet is stable & not wobbling - treat it like constant.

Play with a toy top, watch it spin and maintain its orientation but when the spin rate decreases due to bearing friction, watch the top of the axis describe a cone shape - wobble. Same with a long & heavy bullet, fail to spin it fast enough and it won't be stable unless it is shortened. and lighter. I know this after seeing my 10 twist 6mmAI fail to stabilize 112 grain pointy (long) bullets- look! the bullet hit sideways!

As for that 75 .224 Berger & the Miller Sg estimator:

The math is on the top bar, to the right of Sigma =.

Yes, this bullet can be spinning like crazy, like 400K and only have marginal stability, like 1.35. The length & weight of the bullet is the major factor using the Miller estimator. Looking at the rows & columns for length & weight then seeing how these values are used in the equation explains this. Shoot at something at 11,000 ft altitude on a humid day (19-inch HG) and see the Miller jump to 2.12 with a 5,000 fps MV, For a nice pointy bullet like the 75 Berger .224 VLD, the G7 BC would be a better fit. The Miller is a standard, like most bullet makers and JBM use it

Running the Hornady 4DOF calculator for the similar G7 BC, 75 ELDM with a 9 twist at 4000 the Sg starts out at 1.06 at zero to 1.76 at 500. Hornady uses hi-tech Doppler radar for their ballistic stuff without BC's. I don't have a clue as to US Gov. methods. I would like to see NASA get into this stuff - our tax $ at work.

Bullet makers like to cover all possibilities, so they go conservative on twist recommendations. Would that 6mm 107 Si MK, 9 twist, shoot well at subzero at sea level? My 11 twist .204R shoots 40 Vmax's great at 3675 on a warm day but what would happen at sea level at subzero?