I am having some problems with ballistics calculators. I mean, a lot of problems. A little history first. I am shooting a Savage 10FCP in .308. I have her zeroed at 100 yrds. I don't have a chrony but I have a rough idea of what my MV is. I have BC's supplied by Sierra's website for my bullets. 180gr SMK's. So the data that I input into the calculator(s) is MV, BC, bullet weight, altitude, temp, zero range, sight height, and air pressure. And the calculator(s) crank out a range card. The range card is fairly accurate out to about 500 yrds. At least they are accurate enough to get on steel silhouettes to 500 plus. But the sight in range I have has a steel deer at 600 and the drops put me WAY over the deer. And as I get out to 1000 I am WAY over the target with drops supplied. Now, I know how many clicks I need to use in the real world to get on target at those ranges. Or, at least, I know roughly how many clicks I need to use to get on target at those ranges. 100 yrds = 0 clicks 300 yrds = 22 clicks 400 yrds = 30 clicks 600 yrds = 47 clicks Now my calculator, with correct info is giving me clicks of 0, 20, 33 and 63 respectively. And that was closer than I was a bit ago. (explained below) On a side note, I am a firefighter and am now going through a Wildland Fire Weather course where we were talking about air pressure's. So I used that information to my advantage as a shooter. In short, air pressure will push up 30" of mercury at sea level. You will lose 1" per thousand feet of rise. So at my altitude I will lose 6" of mercury. That puts me at 24". So if I watch the weather and it says barometric pressure is 29.92" I will input 23.92" in my ballistics calculator. Now, that correction puts me closer to the real clicks that I am finding on the range. I thought that was weird since my calculator asks me for both pressure and altitude. I could see a couple clicks here or there since I don't know the actual MV of the bullet but 16 clicks difference at 600 yrds is a lot. And if I change the MV to WAY over what I know it really is I can get my 600 yrd drops right but then my closer drops are way off. What am I missing or are ballistics calculators really that inaccurate?