Altitude vs. Barometric pressure

[nralifer]

Example: RSI Ballistic calculator calculates that at sea level in 59 degree air and 29.53 BP a bullet with a .540 BC at 2700 FPS zeroed at 300 yards will drop 307.9" at 1000 yards.

The same program, same BP, bullet, velocity, zero, temprature, but change the altitude to 5000' and the bullet drop is calculated to be 315.8 with the same barometric pressure imput.

The differance is 7.9"

I always thought that the actual pressure and not altitude changed the bullets perfomance. I know pressure changes with higher/lower altitudes go hand in hand, but thought that altitude in and of itself had no effect on the bullet and just the pressure change with the altitude.

Any ideas?? Is the RSI program correct or is it a glitch?

For actual trajectory calculation I use the actual station pressure measured by Kestrel device at 0 altitude. That way there is no correction necessary because it is the actual pressure you are shooting in. If you want to estimate what a bullet might do at an anticipated altitude then use the corrected pressure function at 1 atm pressure (29.92 mmHg) at that altitude. The trajectory app will correct the pressure at the anticipated altitude. Remember though that this trajectory is only an estimate.

 

[VarmintBob]

Push the button in your ballistic calculator that says "pressure is absolute" then use local atmospheric pressure. Altitude entries will be ignored.

 

[TXTrained]

Push the button in your ballistic calculator that says "pressure is absolute" then use local atmospheric pressure. Altitude entries will be ignored.

For actual trajectory calculation I use the actual station pressure measured by Kestrel device at 0 altitude. That way there is no correction necessary because it is the actual pressure you are shooting in. If you want to estimate what a bullet might do at an anticipated altitude then use the corrected pressure function at 1 atm pressure (29.92 mmHg) at that altitude. The trajectory app will correct the pressure at the anticipated altitude. Remember though that this trajectory is only an estimate.

Excellent Eplanantion & directions. The Kestrel can be pretty intimidating to use by new owners.Especially to people not real Tech Savy. Thanks

 

[WilliamAllen]

Live in WY.use a kestrel a lot my experience has always been at altitude the wind is a bigger worry than pressure.its uphill or down hill .