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Rifles, Reloading, Optics, Equipment
Rifles, Bullets, Barrels & Ballistics
Bullet stability calculation
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<blockquote data-quote="RockyMtnMT" data-source="post: 1132493" data-attributes="member: 7999"><p>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. </p><p> </p><p>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.</p><p> </p><p>Steve</p></blockquote><p></p>
[QUOTE="RockyMtnMT, post: 1132493, member: 7999"] 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 [/QUOTE]
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