You may have seen the terms Zero Height – Zero Offset when working with a ballistics calculator but many people don’t know what that means. Put simply it’s a way of saying to the calculator that you have less than a perfect “dead center” bullseye zero on your rifle. Should that really matter? Why would it be important? How close to dead center do you really need to be in order to get good results downrange? We’re going to discuss some of these questions and a little about the terms so if it’s something you need or want to play with, you know what you’re doing! There’s a lot that goes into ballistics, the software the runs calculations, etc. and we’re all learning all the time. Myself included! So let’s dive into this!
Zero Height – Zero Offset
As I mentioned in the opening this is a way of telling a ballistics software package that you have a less than perfect zero. Say you get yourself all set up and you walk the rifle in on the bulls eye. Maybe you’re really close but you’re still not punching the red off the center so you adjust a click in elevation or windage to fix it. You fire a few more rounds. Now the group has moved to the opposite side of the bullseye. This happens! You can actually get stuck where a perfect zero is between a click of elevation or windage. The question then becomes, how much does this really matter? Do you really need to have an absolute perfect zero in order to get good results downrange?
My experience has been that if you can get within a half minute of the actual bullseye you can call it good. You probably don’t even need to mess with Zero Height – Zero Offset. I try to introduce new shooters to this sport whenever I get the opportunity and this can be one of the first things I’ll see. They’ll get close to the bullseye with their group but not right on it. It becomes a question of where the larger source of error is. Technically, if the closest they get is a 1/4″ high of the bullseye, they aren’t really zeroed at 100 yards. They’re really zeroed at 123yrd or 118yrds or something like that. Then, depending on the cartridge and the trajectory of the bullet, this can be a source of error. Is it really worth burning all the new guy’s ammo chasing a perfect zero when his grasp of the fundamentals may already preclude them from achieving one?
I want to stress from the get go that the effects of this, inside 1000yrds or so is pretty minimal. For years I have gone out and zeroed the gun and called it good if within a half inch or quarter inch of the bullseye. I’ve shot that way well in excess of 1000yrds with success. Targets in tactical shooting tend to be on the large side. Particularly when distances get a lot bigger. However, there are folks that really push the limits. Guys that shoot ELR distances into the mid and high teens or even beyond a Mile need to pay more attention to this. Like anything else in life as the time and distance increase so does the magnitude of any error involved. Maybe you shoot a discipline that requires perfect precision and it makes sense to really dial this in? To each their own!
Other Uses For Zero Height – Zero Offset
Ballistics is a cool thing. I don’t understand or even attempt to understand all the math behind it. There are some really cool things you can do with it when you have the right calculator. There was a time when I was experimenting with different loads for my 308 Winchester. I was curious if it was possible to run two different loads on the same gun without having to rezero the rifle. Would you be surprised if I told you that you can absolutely do this? What if you just want to be able to create a different bullet profile for when your rifle is suppressed versus unsuppressed? When I screw my 338 Thunderbeast can onto my barrel there is a POI (Point of Impact) shift of around 0.7 MILs at 100 yards. I could just rezero with the can, or without it. Why not just create a new profile with the Zero Height – Zero Offset built into it?
The same way you can use these fields to tell your ballistics software that your zero is just a touch off of being perfect you can use it to account for differences in loads for the same cartridge or even whether you have a suppressor on the gun or not. All you have to do is zero it one way and measure the offsets from the zero. I highly recommend you run a 5 round group to do this. Specifically because people possess different skill levels and have all kinds of different gear and other variables in play.
Fire five rounds and determine the center of that group. Then bring out your second load. Maybe you have a different load worked up for hunting than target shooting for example. Use the same point of aim on a new target and fire a second group. Determine the center of that group. Then measure with a caliper how high/low it is from the point of aim and how far left/right it is from the point of aim. Enter those numbers into your ballistics software and it will crunch the math for you! Now when it gives you a firing solution it is altered to account for the difference between what you zeroed with and what you’re currently shooting!
While not a complicated topic, I do think it’s pretty cool. As an example when I was working on load development with the new 115 DTACs for my 6x47L I tried this out. I had zeroed the gun with the new 115s but fired an additional group with my other recipe for 105gr Berger Hybrids. I noted the offset, which was perfectly vertical. That makes sense since the lighter round is moving faster. I entered that into my ballistics software and was able to switch between the bullets without changing the zero on the gun while I was at the range. That’s kinda handy! I typically find a load I like and just stick with one load for one gun. However this has other applications. Like building in the offsets created by suppressors or for guys who hunt with a different bullet than they go target shooting with. Ballistics is cool, right?