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.
Extreme Spread
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?
1. Buy in batches, load in batches
Buy your powder, projectiles, primers, brass, everything in batches, then double check those batches are consistant.
You are going to hear the consistency word a lot here – reducing ES means reducing variance from shot to shot – and that means consistency in components and process.
There can be significant variance between production lot and production lot – and, depending on the manufacturer even within production lot. There is a reason match grade projectiles cost more, and that is often because of reduced variation within a lot.
How far you go down this path is up to you – but many competitors in the US will buy enough components for an entire season (year) in one go – or – also common – once the load is sorted – enough rounds to theoretically shoot out a barrel. Suddenly, when components are hard to get, this makes doubly good sense!
Variance in powder sensitivity and burn rates, weight in projectiles, case capacity in brass, ignition in primers all affect the end result. Reduce the variation right at the beginning and you are off to a good start!
2. Prepare that brass
Even if you have purchased the best brass you can source (and you should) – it still pays to go through it all once and confirm a few things – primer flash holes can be deburred (yes – even Lapua benefits from this), full-length sizing, mandrel neck opening, chamfering and deburring all help to ensure the brass is optimised and consistent right from the beginning. A lot of this is a one-time process – so get it done, then forget about it!
I have a variety of tools I use for this, and tend to do it sitting in front of the TV. It’s not as critical as making sure you charge a case – and it really is just a bit of repetition you need to get done once.
3. Measure every charge
Adam from Autotrickler has a great article on this subject.
Basically – he provides the maths to show that you likely don’t need to be cutting grains of gunpowder in half – but – getting consistent loads to .02 grain weight (which many modern digital scales do) will significantly reduce your SD and therefore ES.
This means you can’t just throw a charge and be good with it – you need to be regularly checking the throw to ensure that the amount of powder you are putting into your case is the same, each and every time (to .02 grain).
Sorry to also break it to you. But your good old beam balance scales are not likely as accurate as you think. Digital scales have come a long way, and with the proper setup, are consistent and accurate.
I am an Auto Trickler fanboy. Happy to admit it. It’s fast, accurate, and well… awesome!
4. Fill up those cases
While you don’t need to be compressing each load to death (and I personally don’t like that as an idea) a 95% plus fill level should be the goal.
If powder can move around and settle in the case – then it provides another area of potential inconsistency.
If you have a load that is leaving a lot of space in the case, then it may be worth considering using a different powder – software like QuickLoad can include case fill as a component of powder suggestion.
5. Manage that neck tension
No. I don’t mean have a regular massage. I mean check that the brass holds the projectile the same. In-consistent neck tension means bullets need differing amounts of pressure buildup before they leave the brass and jump into the rifling – leading, again, to inconsistencies in your load.
Annealing can help with brass hardness, evening it out across the entire batch of brass, as can resizing with a bushing die, and utilising a mandrel in your reloading process.
In my case, I anneal after every firing, neck size to just below my mandrel size (minimising metal overworking) then use a neck mandrel to ensure the inside of each case is the same.
6. Reduce your runout
Runout, concentricity – basically, making sure your bullet, brass, everything are inline with each other.
Reducing your runout is a matter of honing your reloading process. Good dies, good press, properly setup. You likely will never reduce it down to zero – and any resizing we do likely makes this less concentric than brass that has already been fired in your chamber – but minimising runout ensures the projectile engages in the rifling the same way each time.
This helps both with group sizing and the ES of velocity. There is a theory that it also helps with BC ES – but that is a conversation for another time.
7. Be consistent – measure everything
Much of what we talk about comes down to refining your process. And to do that, we need to be able to measure if we are making things better or worse. And that means measuring instruments. If you can measure it, you can isolate it, you can try to refine it, and you can see if you have made things better.
Change one thing at a time, see if it helps, then consistently do that in the future.
Reducing your ES/SD is a gradual process – and it’s important that you document and track each step in the progress. Otherwise, it’s very easy to end up chasing your own tail and not making much process.
You must be logged in to post a comment.