#21
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Stability
Input Data Caliber: 0.204 in Bullet Weight: 40.0 gr Bullet Length: 0.749 in Plastic Tip Length: 0.110 in Muzzle Velocity: 3750.0 ft/s Barrel Twist: 12.0 in Temperature: 40.0 °F Pressure: 29.92 in Hg Output Data Stability: 1.093 26-Apr-17 05:57, JBM/jbmstab-5.1.cgi Stability Input Data Caliber: 0.204 in Bullet Weight: 40.0 gr Bullet Length: 0.749 in Plastic Tip Length: 0.110 in Muzzle Velocity: 3750.0 ft/s Barrel Twist: 12.0 in Temperature: 90.0 °F Pressure: 26.92 in Hg Output Data Stability: 1.336 26-Apr-17 05:59, JBM/jbmstab-5.1.cgi A baro of 26.92 is like 3000 ft elevation. |
#22
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You are correct. I went back on the calculator and it had retained the numbers from yesterday. The tip length somehow changed from my original entry of
.130 to .350. That accounts for the discrepancy. Thanks! |
#23
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JBM Calculator
I am just beginning a new 17 Hornet build. I believe the vast majority of 17 cal barrels employ a 9 twist. Using the following parameters the JBM says a 9 twist substantially overstabilizes a 20 grain Vmax:
Twist: 9 Bullet Length: .538 25.5 mm Hg (about 4000' altitude) Tip length: .130 Temp : 59 F SG: 2.663 The bullet is even overstabilized with a 10 twist according to the JBM calculator OTOH the Berger calculator says a 9 twist has a SG of 1.6, much better |
#24
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Twist rate
I called Berger tech about factoring the plastic tip length into the equation. The person I spoke to was familiar with this topic who said because they do not offer bullets with plastic tips they have not done any testing to determine how a tip might affect the calculations. In general they feel the plastic tip should not be treated any differently because it has no effect on the bearing surface of the bullet. That makes sense to me.
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