Bullet stability calculation

[g0rd0]

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.

I think that the preasure changes that are mentioned are ambient air preasuse not chamber preasures. The air preasure will effect everything from chamber to how a bullet will react. What works at sea level on a clear day may not be too hot on a stormy day

 

[ohiohunter]

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

They were 12+ inch flyers out of a less than quarter minute gun. Several of them over the course of 50+ shots. Conditions were probably the best they ever been. In these conditions I should've cleaned this shoot or at least real close to cleaning it.

Most of my test loads were sub .25" to just over 0.5" at 100yds, I think I had one or 2 open up to about an inch.

Though I am but one case, I now know not to let elevation be the deciding factor for bullet stabilization. Put the calculator on 0 elevation and run that twist without worries.