Bullet stability calculation

[RockyMtnMT]

Ok I need the smart guys to help figure this out. Been using Berger's twist calculator on their web site to calculate stability of bullets we are making. Using the elevation input to show fairly large changes in stability from sea level to 5000 ft elevation. What will not stabilize at sea level has good stability at 5000 ft. According to the calculator. So yesterday we were testing a 177g bullet from a 9 twist barrel at 3000 ft elevation. The rifle was printing oblong holes at two hundred yards indicating to me that we were not stabilized enough to get the bullet on track. So we naturally went to check the twist of the barrel to make sure it was the 9 that was sold as. Turns out to be 9.25". This changes a bit, but according to Berger's calculator it still has a stability factor of 1.23 at 3000 ft elevation. This should be enough to shoot accurately with some sacrifice of bc. So I decided to check JBM twist calculator. JBM's calculator does not have an input for altitude, only for pressure. So I checked the current pressure an used it. JBM says we have a 1.1 stability factor. Still does not explain for sure the oblong holes, but maybe. Pretty marginal. So I decided to look up the pressure at sea level and use that input. It changes very little compared to 3000ft elevation. So I input pressure at very high altitude of 10,000 ft. Still changes very little. The stability factor of the JBM calculator mirrors the Berger calculator as long as you input 0 ft of elevation in the Berger calculator. The test rifle at this point is telling me that the elevation change of stability factor is bogus.

So after all that....Does elevation make a difference in stability of a bullet or not?

Steve

PS

Shot a 160 AB out of the same rifle at the same session and it cut a round hole.

 

[RockyMtnMT]

So now I have been playing with these twist calculators and have noticed that temp changes have a large bearing on stability. A 12% change from 60* to 0* F. So that throws another wrench into the proverbial gear. I would never have thought that temp would have an effect on stability. So as Bryan Litz shows a lower stability factor degrades bc values. So a bullet that stabilizes at 1.4 at 60*F has very little drop in bc. Shoot that same bullet at 0*F and lose 7% of your bc. So what I am learning is that a guy needs to try and keep the stability factor above 1.5 if possible. Especially those of us up north that have such huge temp swings.

Steve

 

[ohiohunter]

Don't completely trust the calculator. It said I could stabilize the 195's at my elevation. Well it seemed to be the case at 100 but when I shot 600 there were frequent flyers of which I attribute to an unstable bullet.

IMO ignore the elevation and temp and stabilize it at sea level 60* or whatever.

Caliber would help, but I'm assuming 7mm.

 

[RockyMtnMT]

Don't completely trust the calculator. It said I could stabilize the 195's at my elevation. Well it seemed to be the case at 100 but when I shot 600 there were frequent flyers of which I attribute to an unstable bullet.

IMO ignore the elevation and temp and stabilize it at sea level 60* or whatever.

Caliber would help, but I'm assuming 7mm.

Yep, sorry I didn't give bullet particulars. .284, 177g, 1.66". Didn't think about. Was more concerned with weather or not elevation makes a diff in stability or not.

Steve

 

[HARPERC]

I "think" you're trying to isolate one factor, while you're experiencing a cumulative effect.

It's why generally, I usually opt for more twist when it's available.

Good luck with your bullet.

 

[RockyMtnMT]

I "think" you're trying to isolate one factor, while you're experiencing a cumulative effect.

It's why generally, I usually opt for more twist when it's available.

Good luck with your bullet.

Not trying to isolate one factor, have discovered two.....I think. Elevation, maybe, and temperature.

So do you think elevation effects stability, or is it atmospheric pressure? The calculators using pressure show little diff in pressure. Calculators using elevation show large changes based on elevation. Both calculators show significant changes based on temp.

Steve

 

[HARPERC]

Did you obtain your inputs at the time and place you were shooting?

 

[FEENIX]

Not trying to isolate one factor, have discovered two.....I think. Elevation, maybe, and temperature.

So do you think elevation effects stability, or is it atmospheric pressure? The calculators using pressure show little diff in pressure. Calculators using elevation show large changes based on elevation. Both calculators show significant changes based on temp.

Steve

Atmospheric pressure changes inversely with changes in altitude.

 

[Beng]

Suggestion: Buy the Applied Ballistics book. How stability is influenced by changes in air density and temperature are explained in there.
Short: The denser the air, the greater the forces acting upon the tip of the bullet will be. Bullets swerve a little, when the tip swerves off the flight path, the center of pressure and center of gravity aren't aligned anymore in the direction of the force enacted upon the bullet by air resistance. This creates a moment trying to overturn the bullet. High altitude and high temperature both reduce air density.

 

[RockyMtnMT]

I understand how pressure works. I don't understand how the elevation change in the stability calculator works. The stability calculator on Berger's web site has an input for elevation. The JBM calculator has an input for pressure. When using the Berger calculator input of elevation has a significant effect on the output data. When using the JBM calculator the input of pressure has very little effect on the output data. I have a rifle being used to test a bullet that has marginal stability for the bullet being tested. When using the Berger calculator and inputting current conditions including the elevation (3000ft) it says that there is a stability factor of 1.23. This should be enough to stabilize the bullet. Use the JBM calculator and input the current pressure of 29.98hg it gives a stability factor of 1.1, not enough to stabilize the bullet. If I use the calculator from Berger and use the same input data except leave the elevation at zero it gives the same output as JBM. So I decided to get current pressure values from around the country from NOAA and use them in the JBM calculator. Using the pressure value at New Orleans (3ft above sea level) then comparing that to the current pressure here. The stability factor is virtually the same. So I found a pressure value for a location at 10,000ft elevation. The output data from JBM changes minutely. As of right now I have a rifle that agrees with the JBM calculations.

So I am afraid that guys are making decisions on bullets that may not be wise. I don't know how many times I have seen guys on here setting up rifles to go hunt out west at much higher elevation than their location using bullets that are marginally stabilized thinking that when they get to 7000 ft elevation the bullet will become well stabilized. It appears that this will not happen. It also appears that if these guys are working loads 60* and headed out west to cold temps they may have a big problem. A 60* degree downward change in temp will make a 12% decrease in stability. If stability is marginal enough it could become unstable. Or according to Litz when stability goes down so does the bc. I have no reason not to believe this, it holds true in my testing. This could create long range misses because of the degraded bc due to lack of stability.

Steve

 

[BryanLitz]

ohiohunter,

Your observation of flyers likely has nothing to do with stability. There are many causes for flyers unrelated to stability.

RockyMtnMT,

The issue your having is that you're using barometric pressure in the JBM input. You mentioned using 29.98 inHg for 3000'. This is the barometric pressure, not an absolute or station pressure. The standard pressure for 3000' is 26.82 inHg. When you tell the Berger program 3000 feet altitude, it's using 26.82 inHg behind the scenes for the calculation.

The weather stations report barometric pressure, which is corrected for sea level. It's not accurate at any given point on the ground (how could it be, the weather station doesn't know how high up you are?) If you're using barometric pressure from the TV, you have to correct that to your altitude.

There's a sticky thread on this site (here it is: http://www.longrangehunting.com/forums/f19/altitude-vs-barometric-pressure-11950/ ) which explains this in more detail. It's a common misunderstanding in ballistics.

Take care,
-Bryan

 

[RockyMtnMT]

Thank you Brian for joining in here. I knew there had to be some error in my input. So when using a ballistics calculator and a hand held meter how do you use the pressure value the meter gives? What stability factor do you think is needed for a bullet to be stabile?


Thanks again

Steve

 

[RockyMtnMT]

I just got out my meter and it gives 26.51 hg. That answers that. I was using the weather station that is in my house which is currently giving 30.00 hg from the outside sensor. That matches the NOAA web site. I had no idea there was a diff in how pressure is calculated.

Steve

 

[BryanLitz]

Steve,

You're on to it now.

Here's another resource that explains the altitude/pressure input. It's the instructions for AB Mobile, but the idea is the same for all ballistics programs and stability calcs:
http://www.abmediaresources.com/ABApp.htm

It's a long document, scroll down to the environment/atmosphere part for the explanation of pressure and altitude.

-Bryan