How does weather and altitude affect power factor

[A62335]

What effect does temperature and altitude have on ammo power factor? Is there a general guideline, or does it differ based on what powder is used? Will one powder gain velocity, and a different powder lose velocity due to the same change in temperature or altitude?

[big_kahuna]

Velocity increases as:

- elevation increases

- air pressure decreases

- relative humidity decreases

Bullets go faster when you have fewer air molecules or microscopic water droplets creating drag on the bullet in flight.

A few years ago I experienced this effect first hand at the NorCal Sectional. I live about 2500' above sea level in the high desert (hardly any humidity), and the match was at Moro Bay near sea level with high humidity. IIRC the 40 load I used was good for about 169 PF at my home range. It went 162 out of my G22 and 166 out of my son's G35 at the match. While my son barely made Major PF, I went minor because I didn't build enough cushion into the load for the air density/humidity differences.

Edited December 21, 2009 by big_kahuna

[Alan550]

Here are a couple of sites that address some of what you want to know:

http://www.sengpielaudio.com/calculator-speedsound.htm

http://www.aerospaceweb.org/question/atmosphere/q0112.shtml

The 2nd one addresses the speed of sound and how altitude affects it in case you need to keep a load either sub- or super sonic over a given distance.

Alan~^~

[RogerT]

And to makes things worse, powders react differently to temperaure changes, one powder might drop velocity with increased temperature where another powder increases velocity in higher temperatures..