Getting Your Inputs Squared Away Part: 2
Ok you went out and bought yourself the Kestrel 3500 and you got it set up like we discussed in the previous article. Now, go outside and measure your barometric pressure. Now take a drive to where you shoot long range. Get a pressure reading there. Now you’ve got a single set of data points. You’ll want to do this a few times so you can get a running average and the daily standard deviation and extreme spread and coefficient of variation for the places you shoot. You can use the Shot Log & Statistics page of Ballistic_XLR to find the statistics for your BP measurements. Once you have your statistics you’ll be able to make an intelligent decision about what base BP setting to put in your charts.
I live about 80feet above sea level and right next to the coast so my BP is usually sitting pretty high. Day to day I see 29.5, 29.3, 29.8, etc… so it hovers around the middle of the 29InHg mark with a little bias toward the upper end of that. Living next to the coast though increases humidity which drops air density. So I set my tables up usually for 29.3InHg. That takes care of my average local humidity. When I go to the Mojave Desert I end up at 2800ft altitude which drops my BP to 27.7 or thereabouts and it crushes the humidity. Less humid air is more dense. So for that situation I’ll set up my tables for 27.8. What I’m doing there is doping the BP ever so slightly to accommodate large swings in humidity by adding or removing some air density which is exactly what humidity does to ballistic trajectories. A .1InHg increase or decrease in BP should be sufficient to take care of the full swing from 10% to 90%. I usually use a .1 change in BP for every 40% of humidity. So if I’m in the desert and it’s 20% humidity I’ll add .1 to BP. If I go out and find I’m shooting in 100% humidity then I’ll drop .1InHg from my BP.
So to sum up, set your BP to a solid representative BP for the area you shoot in. A decimal change in BP (using inHg) or a 5HPa change (in HectoPascals) is enough to throw off the shot at extreme range. At shorter ranges it’s the difference between hitting the bulls-eye and just being on-paper. Once you’ve got your BP nailed you’ve got your drop figured out.
Stay tuned for Part 3.