My last post emphasized the importance of a predictable zero to hit probability at a variety of ranges. It was only slightly less important to the task than raw marksmanship skill and wind calls.
Breach Bang Clear recently put up an interesting article that got me thinking about the subject. I’ve also seen some forum postings lately asking about zeroes. One post in particular assumed that zeroing a red dot sight on one rifle and then moving that optic to another rifle meant that the optic would have the same point of aim/point of impact (POA/POI)- they wanted to know the best QD mount for doing this. Other users corrected this person, but it is reasonable to think that many relatively new shooters out there don’t understand this concept.
First off, to answer the question I saw in the forum, every zero is unique to the combination of sighting device, rifle, and the ammunition. When you see optics mounted on QD mounts, the intent is not that the optic can be moved from one rifle to another while maintaining zero. Rather, as in my case, the user might have more than one optic zeroed for a given rifle/ammunition combination and wants the ability to swap optics as needed. A good QD mount means there is little to no shift of the zero as the optic is dismounted and remounted.
I don’t want to rehash what Breach Bang Clear wrote. So I’ll simply that part of this post by saying that if you want to maintain maximum accuracy, then you should rezero every time there is a significant change in the conditions that your previous zero was established in. What kind of conditions? If you change the type of ammunition, or even a different lot of ammunition from the same manufacturer, you might need to rezero; if the temperature or barometric pressure changes (including changes in altitude), you probably will need to rezero; if you rebarrel the rifle, or move the optic to a new rifle, you will need to rezero; if you change the torque values on the mounting screws, you will probably rezero.
What is the cost of not being this dedicated to maintaining a solid zero? Well, that depends on your required level of precision. I cannot say how much your zero might drift if any of these conditions change. Some will have more of an effect than others, but all will have an effect.
So what else is there to this “predictable” zero? Your zero is not just your chosen POA/POI. If you need to shoot at different distances, then you should also know what the difference will do to your POI. For instance, the BDC markings on an ACOG are not a guarantee. A predictable zero means that you take your optic/rifle/ammunition combination and actually shoot at each of the ranges on that BDC to see where they actually hit. Once you know that, then you have a more predictable zero.
Also, keep in mind that ballistic calculators (as handy as they are) aren’t much better than the BDC. It’s still a notional idea, and you will have to go out and see for yourself what your bullets actually do during flight. That is why generations of precision shooters have kept data books and logs of every shot fired in a variety of conditions. This, rather than just plugging numbers into JBM or their chosen ballistic app, allows them to predict what their shot will do from experience and real data.
I’ll admit that I’ve been lazy in this regard. For the most part, the weather where I live doesn’t change all that much. I really only zero when moving optics from rifle to rifle or when I change ammunition. I also keep track of how much of a change it is for each. For instance, I know it is a 5 MOA increase in elevation POI when switching from ADI 69gr SMK to Privi 55gr FMJ with the 20″ musket. I don’t quite have the talent to worry about much more specific than that. A change in lot of ammo or air density might affect a change in POI that is less than my natural ability, so I probably wouldn’t notice. But an F-Class shooter…yeah, they’d know.