In this article, I am going to talk about the definition of the terms, how important they are (or are not) and what can be done in our reloading process to reduce the ES and SD, if it is something that would actually be of use to us.
What are we talking about?
ES and SD are mathematical terms that relate to a set of figures, or values – in this case, velocity. Specifically, the extreme range (ES) of the set of values as well as the expected variation and uniformity (SD) of that set.
ES is the simple one. It’s the spread between the lowest and highest value in a set of numbers. For example. With a set of numbers that range from 1 to 10, no matter how many numbers are included in between the two – the ES is always going to be 10.
In terms of shooting – say we had a string of five shots, the lowest velocity was measured at 2750fps and the highest was 2800fps. That is an ES of 50fps. Which is a good, not phenomenal ES spread.
Say we had 2750 and 2780 – that’s now an ES of 30fps. Which is really good. If you start seeing figures under 20 – well – you likely don’t need to be reading this article!
How does ES effect the impact?
Well – more correctly understood – how does different velocity affect the impact of a round? Simply put, velocity differences result in different vertical points of impact. Higher velocity, higher impact, lower, lower. This compounds the further out you shoot, because it’s all about angles!
Now. It’s is actually very easy to figure out how much of an impact this makes to you personally – and instead of putting out a pile of graphs, I am going to encourage you to actually open up that ballistic solver, and put some numbers in to see how much it affects your shooting, your rig, your solution.
If you have run your load over a chronograph – you will already know your average velocity (and for that matter, the ES and SD of that test). Remember, the more data (more rounds shot) in your test, the better when it comes to the maths relating to sample size.
So – you have your average, and your ES. For the sake of a simple comparison – half that ES figure, and subtract it from your average. Find out what drop that results in at the distances you want to be shooting at. Now, add half to the average figure. The two resulting ballistic solutions are the lower and upper brackets as to where the round may hit in a string of fire. If it is way smaller than the target size you are likely to be shooting at (allowing for some margin of error) – then stop worrying and start shooting!
However, if it is enough to potentially put you off the lower or upper edge of a plate, or outside a ethical kill zone on an animal – we need to either reduce our effective range, or reduce our ES.
And what is SD?
SD is a little more complicated. It is essentially the mathematic likelihood that the given data point is going to deviate from the average. In short, the higher the SD, the less consistent the velocity spread is and the more ‘all over the place’ the impacts are likely to be.
It’s harder to present a practical example of how your SD will affect your shooting. So. Lets just consider this – as we work to reduce our ES, our SD will likely reduce as well. And SD in the single digits is what we are striving for.
Low ES/SD doesn’t mean smaller groups.
Important to note – reducing ES/SD doesn’t reduce group size – and a small group size isn’t an indicator of low ES/SD – in fact – many small groups still have a higher ES. Which – again – depending on your use may or may not be an issue.
If you bush pig shoots tiny groups, but has a high ES – so what? You likely won’t notice any difference in the shooting you do. Vice versa – a 1 MOA load with a tiny ES, may actually serve you better than a 1/4 MOA group size with a large ES, once you start shooting to distance. As always – it depends on what your intent is. Define that first before you start chasing things you might not even need to chase.
In short – don’t necessarily listen to the advice of bench rest shooters who are shooting for the smallest group size, and the same distance each time. It’s a different discipline!
So. How do I reduce my ES?
Ok. So working on the assumption you want to reduce your ES. What methods do we have?
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In this article – seven simple steps you can take to start reducing your ES/SD right now!