Density Altitude Discussed

One of the more interesting questions we get around here is about density altitude as it pertains to external ballistics. So, we thought it would make a good shoot-the-shit topic for this Friday. In this post we want to shed some light on the concept of density altitude as it pertains to external ballistic performance.

External environmental parameters influence the efficiency of a projectile moving through the air for a given ballistic coefficient and projectile geometry. Different bullet designs perform differently under similar conditions and all bullet designs exhibit flight paths influenced by environmental conditions.  The impact of that influence increases with range to the target. For example, police snipers  don’t usually concern themselves with environmental conditions when they operate within their usual engagement range of 53 yards; however, they all understand environmental parameters and will employ them as range to the target increases. A one inch error at 100 yards translates to a three inch error at 300 yards and six inches at 600 yards.

In determining a firing solution, precision shooters must take temperature, station pressure, humidity and altimeter into consideration. Prior to the integration of ballistics software with weather meters, like the Kestrel NV4500 family, ballistic calculations involved manual entry of environmental data into a separate device like a PDA or smart device to arrive at a firing solution. This procedure is both tedious, time consuming and what happens when your ballistic calculators gets run over by a Humvee  – enter density altitude.

Density altitude is an aerodynamic concept used extensively in aviation. For example, on a hot and humid day, I know that I’m going to have a longer run down the runway before rotating. You may have heard the term “flying by the numbers,” here’s what that means. It means that you adjust aircraft attitude and indicated airspeed for specific values under specific conditions. For example, rotation air speed will vary with fuel and weight. The best rate of climb speed is that airspeed that results in the best vertical gain for horizontal travel. This value varies under different conditions. Yet another performance factor is fuel consumption, it too will vary with density altitude. So, it follows that a projectiles performance will vary with density altitude.

Pencil and paper computation of density altitude is not trivial needing a  couple of conversions and requires that you have the following data: temperature, station pressure and dew point. All commercially available weather meters capture that data and automatically calculates density altitude. Once you have your density altitude value from your weather meter,  you’re ready to get your firing solution from commercially available cards.

Density altitude cards, also called estimators are available in sets grouped by ammunition type, meters or yards, MRAD or MOA. Using these tools is easy and eliminate the need for a separate ballistic calculator.

I use cards as a backup against device failures due to battery exhaustion or device damage. They work great and the firing solutions are accurate. If you are a precision rifle shooter having a set of ballistic cards is strongly recommended and highly desirable.

As always we look forward to hearing from you, or if you want to change the subject all together, well that’s cool also– it’s a shoot-the-shit.

Have a great weekend everybody, be safe and have some fun with your buds or family!