I hear the comment quite regularly – ‘I got in real close to the animal – but just couldn’t seem to get the bullets to go where I was aiming – I was so close, I must have had to aim lower, right?’
Well, no. If you aimed lower, there is a good chance you put a round under the animal – or worse still – into a leg.
But why? Oh. Maths!
Ok. Here it is in a nutshell. Mechanical Offset.
Also referred to as Scope Height over Bore – mechanical offset is the simple fact that your scope sights sit higer than the bore of your rifle, and at close distances, that offset is enough to make quite a difference.
While generally spoken about in reference to CQB shooting and AR’s – it’s a very real consideration for any shooting at close distances – like those often found in bush hunting in New Zealand.
It’s also part of the reason that some people still prefer the simplicity of iron sights on a rifle, and, why we used to have an ongoing obsession with getting the scope down as low and close to the bore axis as possible. This includes Leupolds truly unique take on the dilemma. The VX-L – any guesses what the L stands for?
Basically, the lower down, closer to the bore you can get a scope, the less the mechanical offset is, and the less you need to account for it.
When you zero a rifle – you are not aligning the line of sight (your scope) with the end of the barrel. If you did, you would be ‘zeroing’ the rifle to 0 meters.*
Instead, we are picking a nominal point (100 meters or yards) and setting up the firearm so that the bullet trajectory and the line of sight align at your chosen distance. From there, we have a known, that we can allow up (or down) from when the distance to your target varies.
Your bullet will actually pass through your line of sight on the way out to the target and while it is ‘technically’ dropping from the moment it leaves the barrel due to gravity, relative to the ground, it heads up, then back down to land at your ‘second’ zero.†
These days, more and more people are focusing on the ‘long-range’ of things – wanting to shoot further and further out. But the reality is, for newer hunter – and, for that matter, simply most hunters – most shots will be taken under 100 meters – often, in the central north island for example – under 50.
So. What is happening at these closer distances?
Well – this is where that often misunderstood, and often unfilled field in your ballistic calculator comes in.
Scope Height Over Bore
The term means what it says – what is the scope height over bore – specifically – the middle of the optical scope tube, to the middle of the rifle bore, or barrel.
Instead of another diagram here – lets have a video, shall we?
I just use the measuring tape method. I have run the numbers, and the difference doesn’t make enough difference for me to bother.
Talking of numbers. Let’s punch some figures into Coldbore and see what is actually happening. Feel free to go out and shoot the distances and actually see – but maths is one of those things that really does work the same on paper and in reality. Lots of things would break if that wasn’t the case.‡
The Maths (does it drop or rise?)
I say maths, but really, it’s a pile of graphs as the result of some maths I got an application to do for me.
Some details of the example we are using.
I used Coldbore as the software, modelling the Sako Powerhead II .308 factory ammo. This has box figures of 168gr projectile weight, 2559fps muzzle velocity and a G1 BC of .470 – I loaded this into the system and ran the figures.2
The first graph shows us the drop, from the muzzle, out to a 100 meter zero for two different scope over bore heights – 3.5cm, representative of many hunting rifles, and 6cm – much higher and closer to modern, long-range tactical scopes on high rings that are being put together.
Basically, as expected, both start at the expected offset, (3.5, 6cm) rise over the line of sight and then drop back down into the 100m zero.
We will note that the 3.5cm offset gets up to the line of sight (where your crosshairs are) at 40 meters, while the higher offset of 6cm takes until 70 meters until the projectile is at where you would be aiming.
In both cases, if the projectile is still below your line of sight/aim – what are you going to need to do to hit the target where you expect? Aim high. Now it might become clear why a lower mechanic offset for the purposes of close bush hunting might be desirable. The less to offset to start with, the closer the projectile is going to be to the point of aim, especially at those super close shots.
Some of you, no doubt are thinking – ‘well it’s only 1.4cm at 20 meters, what’s the issue?’ – and, to a certain extent, you would be right – if your scope has a 3.5cm offset. Many modern rigs are more than that. And, we are assuming you hit exactly where you aim at, while standing, in what, at that distance is potentially a snapshot, and your zero is perfect… and.. and… and..
At best, you just hit a little low. Oh, but wait – it’s so close you decided to shoot between the eyes. So a couple of centimetres would be where? The jaw potentially?
Assuming a 100 meter zero.
OPBZ, MPBR, walking zero and it’s effect
So, all of this so far has been based on a 100 meter zero. What I would be walking around with most of the time. However, some folk like to run an MPBR, or, for those just doing what everyone else does – zeroing at 200 yards, because, well… that’s what I was told at the Deerstalkers!
But, ballistics are affected by such things – but how?
That’s how! Because we are essentially stretching out the ballistic curve requiring it to come back down to our line of sight later, we also need to lob (it is a 308, after all) it up higher to do so.
So much so, in fact, that the max ordinance, where the projectile is at it’s highest (prior to zero) will be at around 8.1 cm – this is based on an OPBZ calculated using a 10cm target diameter. This is at just after 100 meters. Look at it this way – at 100 meters, you are shooting 7.9cm high.
Some data, zoomed-in – focusing on below sight line travel now. Our first crossing of the line (with a 200m zero) occurs at 20 meters, instead of 40 like a 100m zero. You could argue that this is good – as it means you are closer to point of aim, point of impact at such close ranges. If you chose to just focus on one aspect while ignoring the obvious issue that the rest of the way out to 100 meters you are waaaay higher.
Like most things relating to firearms – I am always going to encourage you to figure out the details for your specific firearm – hopefully, this article helps explain a bit of how this works – and you can apply it to your specific situation. The amount of under/over is going to be specific to your rifle and scope setup, as well as what the calibre you are shooting. A ‘flatter’ shooting round will mean less to worry about – but – you might also then be tempted to run an OPBZ – which increases it in turn.
Learn your gear, learn how this all works. Make better decisions.
- *Which, incidently, breaks a lot of ballistic calculators when you try to figure out where the round would come back down through your line of sight.
- †This is partly where things like an AR ‘battle zero’ comes from – or people zeroing at 25m and calling it good for 200m. Protip – it’s not.
- ‡Yes I know there are exceptions, no, they are not relevant to anything we are discussing here.
- 1.Wilson W. Mechanical offset review for optics. PoliceOne. https://www.policeone.com/police-products/firearms/accessories/sights-scopes/articles/mechanical-offset-review-for-optics-RCFj5R4Wd7i0Zpal/.
- 2.Powerhead II. SAKO. December 2014. https://www.sako.fi/cartridges/sako-cartridges/powerhead-ii.