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Long Range Hunting & Shooting
new twist on b.c.'s
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<blockquote data-quote="JBM" data-source="post: 11995" data-attributes="member: 1969"><p><strong>Re: new twist on b.c.\'s</strong></p><p></p><p>I may not have answered this, so I'll follow up my own post...</p><p></p><p>You can pretty easily calculate the CD of the bullet for "small" changes in velocity and time using the formula:</p><p></p><p>CD = dv/dt*(8*SD)/(Pi*Density*V*V)</p><p></p><p>where dv is the change in velocity, dt is the change in time, SD is the sectional density, Pi is 3.14..., Density is the air density. Watch the units, the SD must be lb/ft^2. This formula is just the differential equation and we approximate the derivative (rate of change) of the velocity with the the ratio of the measured changes. This is why it is only valid for small changes.</p><p></p><p>To find the BC, you would have to interpolate into a table of CD for the the G1 projectile and then use:</p><p></p><p>BC = SD*CD(G1)/CD</p><p></p><p>where CD(G1) is the CD of the G1 projectile for the mach number (velocity over the speed of sound). The ratio of CDs is just the "form factor".</p><p></p><p>Calculated this way, you wouldn't have to do any integrals, but again, it's not really valid if you want to measure velocities and times many yards apart.</p></blockquote><p></p>
[QUOTE="JBM, post: 11995, member: 1969"] [b]Re: new twist on b.c.\'s[/b] I may not have answered this, so I'll follow up my own post... You can pretty easily calculate the CD of the bullet for "small" changes in velocity and time using the formula: CD = dv/dt*(8*SD)/(Pi*Density*V*V) where dv is the change in velocity, dt is the change in time, SD is the sectional density, Pi is 3.14..., Density is the air density. Watch the units, the SD must be lb/ft^2. This formula is just the differential equation and we approximate the derivative (rate of change) of the velocity with the the ratio of the measured changes. This is why it is only valid for small changes. To find the BC, you would have to interpolate into a table of CD for the the G1 projectile and then use: BC = SD*CD(G1)/CD where CD(G1) is the CD of the G1 projectile for the mach number (velocity over the speed of sound). The ratio of CDs is just the "form factor". Calculated this way, you wouldn't have to do any integrals, but again, it's not really valid if you want to measure velocities and times many yards apart. [/QUOTE]
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