Formula for Velocity Calculation

[kyreloader]

I have read on this forum before about a formula for figuring velocity of a bullet compared to another bullet in same barrel/caliber keeping pressures similar. For instance, in my 6.5x55 BJAI, if 140g Hybrids max velocity at safe pressure in my rifle is 3000 fps, how could I figure the velocity of the 155g Cauterucio at the same pressure?

Another example, my 180g Hybrid load in my .284 Win shows pressure at 2860 fps, what would be my calculated velocity of a 151g bullet in the same barrel/chambering at the same pressure?

I seem to remember a thread where Bryan showed the formula when discussing 30 caliber bullets, I just can not seem to find it.

Any leads?

 

[dmoran]

Under my thread called "Excel Worksheets", I have a downloadable Excel called "Velocity Prediction"
The formula I use is: (Mv * SQRT(Current Bullet Weight / Next Bullet Weight))

URL: http://forum.accurateshooter.com/threads/excel-worksheets.3891679/
Direct Link: Velocity Prediction.xls
Donovan

 

[Ned Ludd]

The formula most commonly used to estimate this in [a relatively crude way] is based on the formula for kinetic energy: (one half) x (mass) x (velocity squared), or 1/2mv*2. Set up the equation for equal kinetic energy so that you have the velocity and mass for bullet 1 on one side equals the velocity and mass for bullet 2 on the other side. The "1/2" on each side cancels out, leaving (mass #1) x (velocity squared #1) = (mass #2) x (velocity squared #2), or mv*2 (#1) = mv*2 (#2). Plug in the mass and velocity of bullet #1 and the mass of bullet #2 and solve for velocity of bullet #2. In your example above:

(140 gr)(3000 fps*2) = (155 gr)(V2*2)

Multiply (3000 squared) by 140, divide by 155, and take the square root to get V2 = ~2850 fps.

This is only a crude estimate as bearing surface length, powder burn rate, pressure, and other factors will not be exactly the same with the two bullets. However, it can be a reasonable estimate for predictive purposes.

 

[kyreloader]

Thanks guys, exactly what I was looking for.