Bullet stability

[scpaul]

If I have a bullet that is close to stabilizing, say it needs 9" twist and I have a 9&1/2 twist barrel would increasing the velocity (bullet rpm) help stabilize it? I realize that the bullet will still have the same rotation to distance ratio but can it be fooled?

 

[bultinkle]

Increasing velocity will increase the spin rate

MV X 720/Twist Rate = RPM

 

[Wedgy]

Higher altitude will also help but marginal stability is ok for shooting targets and steel but the bullet probably won't perform on game very well.

 

[Muddyboots]

Higher altitude will also help but marginal stability is ok for shooting targets and steel but the bullet probably won't perform on game very well.

+1!
Terminal performance highly affected by the SG and far more important that how pretty holes are in paper.

 

[Hand Skills]

I realize that the bullet will still have the same rotation to distance ratio but can it be fooled?

The bullet cannot be fooled, only people.can be fooled

The science to twist rates

Is there any science to what twist rate is correct or is the best you can do is make an educated guess? I currently shoot a lot of 338 cal and a 1in 12 works great for 250s. I know in most calibers a 12 twist is way too slow. I am getting ready to build a gun in .243 cal and it looks like...

www.longrangehunting.com

 

[just_jon]

Check out Berger's stability calculator. For example: 7mm 180 Hybrid Target requires a 1:9 or faster twist. Using data from my 280AI (2975 fps, 60F and 800' elevation), a 1:9.25 twist is marginally stable at 1.45. To increase stability to 1.5 and achieve max BC requires an increase to 3290 fps. A factor of 1.45 decreases BC by 2%, thus the concern with terminal performance.

 

[scpaul]

In the formula for bullet rpm why is 720 a constant?
If I'm not mistaken our elevation is 300+ feet at the statehouse. My elevation is lower.

 

[bultinkle]

In the formula for bullet rpm why is 720 a constant?
If I'm not mistaken our elevation is 300+ feet at the statehouse. My elevation is lower.

It's the inches in a foot (12) multiplied by 60 seconds in a minute for rpm

The long form is

MV x (12/twist rate in inches) x 60 = Bullet RPM

 

[scpaul]

thank you, I couldn't figure out why it was constant and knew that there was a reason.

 

[MagnumManiac]

This discussion always baffles me…BC, and therefore stability, is not a constant number depending on velocity as a large part of the equation, but this is rarely discussed.
My custom solid brass CNC turned bullet at 323 grains in 338 varies BC so much over testing that I don't even bother writing it down anymore. 10% swings for this bullet are quite normal just with atmospheric differences…altitude swing it even more.
The only sure way to know anything is to shoot it at the range you want it to hit at. This is another fact that most don't even do on a regular basis. This particular cartridge and bullet are quite capable to 2,000 metre steel shooting and would take game at a mile…not that I do because there is no open country that big here to shoot those ranges, but you get my point I hope.
I am open to discuss this, just be aware that the time difference will mean I cannot reply straight away.

Cheers.

 

[FEENIX]

If I have a bullet that is close to stabilizing, say it needs 9" twist and I have a 9&1/2 twist barrel would increasing the velocity (bullet rpm) help stabilize it? I realize that the bullet will still have the same rotation to distance ratio but can it be fooled?

Depending on the SG calculator, i.e., https://bergerbullets.com/twist-rate-calculator/, MV, temp, and altitude are some attributes that can affect the SG.
https://www.jbmballistics.com/cgi-bin/jbmstab-5.1.cgi uses other factors like bullet tip inserts length and pressure.

 

[ObiWanKannoli]

This is your friend:

JBM - Calculations - Stability

www.jbmballistics.com

 

[scpaul]

I understand that stability changes with several variables. I have an old RCBS program that allows 4 different bc's to be entered at various velocities down range. I was just asking if I could fudge the twist a little. I'm just above 0 elevation probably about 250 ft. and understand that is another limiting factor.
Like I said, I understand that no matter how fast you push the projectile that the ratio of revolutions per given distance is the same. Can this be fudged by any means other than the projectile length? According to Barret, he designed a projectile to help the Barret .338(?) To improve stability and maintain velosity for greater distances.

 

[Mikecr]

I realize that the bullet will still have the same rotation to distance ratio but can it be fooled?

No, not really.
Shooting through air is similar to shooting underwater. The velocity itself just doesn't make any difference to the bullet wanting to waiver or tumble.

This discussion always baffles me…BC, and therefore stability, is not a constant number depending on velocity as a large part of the equation, but this is rarely discussed.

BC and stability are not directly tied, and only loosely similar.
If you had a matching drag curve for your bullet, BC would not change while traveling downrange.
If actually talking about bullet drag changing downrange, this is not easy to calculate. While a drag coefficient typically goes up with slowing velocity, drag itself (which is squared with speed) is going down.

It happens that Sg goes up downrange, because revolutions decrease at a lower rate than velocity. The 'effective' twist rate goes up.
Once the bullet is above 1.5Sg (fully stable) then little variances to stability from changing bullet drag doesn't matter.
It's when drag hits hard that strong stability is challenged. Like at transonic and animal impact.