Range Report Altitude Change and it's Effects on FPS

[gbernardski]

I sighted my 6.5x284 in at 5200ft in elevation and it was steady at 2940fps out the barrel. So I went on Vortex's sight(I'm using a vortex FFP PST) and used their LRBC program and punched in everything and got a solution out to 1000 yards, which was pretty dang cool. I live CO so my altitude can change a lot depending on where I'm shooting.

So...when I go to change my altitude from 5200 feet to 6500 feet will I be fine to put in 2940fps or will I need to put some assumptions in for less altitude or more altitude depending on where I'm shooting?

 

[Ledzep]

Re: Altitude Change and it's Effects on FPS

Altitude changes won't affect your muzzle velocity (enough to worry about).

What you're really looking for is air density, or barometric pressure. The higher up you go, the less air pressure, the less dense the air is. That has an affect on the air resistance of the bullet.

Ideally, you'd have a Kestrel tell you your station pressure (air pressure where you stand), and would plug that into the calculator.

Altitude alone is not very accurate. Weather conditions can change air pressure at the same altitude. Altimeters read barometric pressure to give a guess at altitude, but they must be calibrated often based on reference stations or else they'll be off several hundred feet because the weather has changed.

 

[Graham]

Re: Altitude Change and it's Effects on FPS

The History Channel got Rob's shot wrong: The bullet doesn't travel faster because it's at a higher altitude in the mountains of Afghanistan.

Ammunition temperature is what has a measureable effect on The velocity.

For ballistic performance, Denesilt Altitude is what you need to know.

 

[gbernardski]

Re: Altitude Change and it's Effects on FPS

Thanks for the info!

 

[kraigWY]

Re: Altitude Change and it's Effects on FPS

Here is a chart you can use ICAO (International Civil Aviation Organization) Standard Atmosphere Chart;

http://www.bimmerboard.com/members/johnhstein/original/ICAO%20STD%20Atmos.pdf

 

[hdfoxguns]

Re: Altitude Change and it's Effects on FPS

I like Strelok. It is an app that I down loaded for my android phone. What I think is neat about it is you enter you zeoring conditions allititude and temp and you get a drop chart for that information. Then when you go to a location that is for example 5000 ft elevation instead of the 2800 you zeroed at you enter that elevation and the temperature and it adjusts for the differences. I have used it only one time in a match and the elevation was around 1000 feet diferent and the temp was 20 degrees cooler and all my setting were spot on. I know one experience with that small of a change in evlvation might not prove much, but it seamed to work great. Anyone else had any similar experiences with this program?