Re: density altitude
<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 1911.it</div><div class="ubbcode-body">Dumb question - Does density altitude really make that much difference when calculating the holdover value? Is the variation greater depending upon starting altitude? I had never heard of density altitude before seeing it referred to on this website. (I'm trying to justify the purchase of a Kestrel 4000!)</div></div>
That depends on how far and what you're shooting. Lots of things affect ballistics, some will matter for a particular situation and some won't. For example, if a whitetail buck steps out from cover at 85 yards, I'm not gonna bother calculating potential bullet spin drift before aiming. On the other hand, if you aim heavy artillery for 30-mile shots you better be calculating Coriolis effect every time.
Density altitude is a nice mathematical summary of three important air effects. (Station air density, temperature, and humidity.) A Kestrel will give you that one number, clean and quick.
For any particular bullet type and muzzle velocity (calculated by testing a particular load from a particular rifle), density altitude will affect the bullets trajectory a particular way; maybe a little, maybe a lot. The lighter and slower the bullet and the longer the range, the more it matters. You shouldn't have to ask if it matters, because you will see exactly what effect it will have after you've done all the calculations and tabulated them. You'll see what effect a 10,000-foot change in density altitude will have on the impact point of your ammo at whatever ranges your interested in.
I've found in hunting, with my gear it starts getting important at about 500 meters, but I rarely shoot animals that far away. (It's more challenging seeing how close you can get.)
Furthermore, if you're calculating holdover value that precisely, then you shouldn't be calculating holdover value. You should already have little tables prepared that give you range clicks from a base (say, 100 meters) in 100 meter increments and windage clicks from zero for 10 km windspeed increments. One different card for every 1000 feet of density altitude, all of which are specific to one particular handload and rifle.
If you want, you can also include average Coriolis effect and spin drift in the prepared click calculations, whether or not they're significant. That's because it doesn't cost you anything more in the field to have that tiny bit more accuracy available. Ammo temperature makes a difference too, but it's best to just use the next density altitude card up or down if the ammo gets real hot or real cold.
So, you prep and bring a half-dozen little range cards with you, and just choose the right one for that hour, based on what density altitude the Kestrel tells you and how warm the ammo is.
You use the same Kestrel to tell you exactly what the crosswind is at the muzzle, and tweak that for SWAG downrange wind effects. Estimate or laser the range, look on the card for range click count, then crank the range turret. Look on the card for your wind speed click count, crank the windage turret, and you're good to go.
I think reference cards and a little Kestrel with new batteries is very light and reliable, so I use that system.
