Getting Your Inputs Squared Away Part: 7
We touched on some of these before but we’ll go further this time.
Latitude: Within a couple degrees is fine for most people. As you move away from the equator it gradually becomes more important. We know that you won’t be sniping at 0 degrees latitude so don’t put that value in. It’s liable to cause some weirdness if you do. For me, I bounce back and forth between 33 and 40 degrees. For the most part I just leave mine at 35 or 37. This is a big part of Coriolis drift calculations so don’t guess. Approximating is fine. Guessing is not.
Zero Range: This is either in yards or meters, which ever you’re using. You don’t need to denote yards or meters, just put the number of whichever of those you’re using in there. If you’re zero’d at 250 yards then put “250” in the box.
A further note on zero range: I try to maintain a 100 yard zero on all of my guns. The scopes I use have enough “up” in them to get me out to the ranges I shoot. This means that I often use canted scope bases and eccentrics in my Burris Signature Series rings to add in extra minutes of angle. The side effect is my scopes are frequently set with the turrets adjusted 2/3 of the way toward the bottom. Sometimes more on the punkin’ slingers. Ideally you’ll want your optic in the center of its mechanical range while at zero for optical reasons. This is also a reason I use fixed magnification scopes and not adjustables. Fixed power usually nets a little more adjustability since you don’t have another set of lenses taking up space. If you can’t zero at 100 that’s fine but it’s a lot harder to actually do. A lot of people will go 2.5″ high at 100yrds/meters but what they have to do then is look up their drop to estimate a nice round number of cubits to call their zero. There’s so much guesswork in the “N number of inches high” zero’ing method that I recommend against it. It’s fine for hunting, not for target work.
Ballistic Coefficient: This is the most critical element to get right. You need to find the exact bullet you’re using either on the manufacturers website or in the Projectile Database tab of Ballistic_XLR. Many companies make multiple bullets in a given style with widely differing ballistic profiles. Be sure you get the right one. If you put in bad data and get bad results don’t blame the math. Users of sierra bullets need to pay special attention to velocity regimes when figuring out what your BC should be. Most other manufacturers give their BC’s as an average that approximates what you’ll see and it’s been my experience that this is a much easier way of doing things if in theory less accurate. Also note that Ballsitic_XLR ONLY supports G1 ballistic coefficients. If you put in G7 data it just won’t work right. If you’re not sure, a quick phone call to your bullet manufacturer will quickly provide accurate information. They’re all very helpful, especially Sierra and Berger.
Bullet Weight: This is the bullet weight in grains. It’ll be listed on the box the bullets/cartridges came in. Just the numbers go in the input box. No letters. This is important as heck for ballistic calculations to come out right.
Bullet Diameter Inches: This is the bullet diameter in inches. 5.56mm = .223, 7.62mm = .308, etc… Only put numbers and decimal points in there. No letters.
Bullet Length Inches: You can measure one with a micrometer or just look it up in the Projectile Database tab. Just like everything else this is pretty important. It governs much of the spin drift numbers so you’ll want to get it right. This is a decimal inch number. 1 3/4 inches is 1.75. Don’t put ” marks or letters in the box.
There you go. We’re done with the basic inputs. Metric users will probably want me to go over this in more detail and there are things I didn’t discuss like what happens when you have MRAD reticle and MOA turrets configured and other interesting edge cases that are actually pretty common. Those topics will be covered in future articles and via support.
Don’t forget to check the download link periodically for updated versions.