Mechanical Offset
Also referred to as Scope Height over Bore – mechanical offset is the simple fact that your scope sights sit higher than the bore of your rifle, and at close distances, that offset is enough to make quite a difference.
By definition, mechanical offset is the distance between the sights and the bore and has an effect on the relationship between point of aim (POA) and point of impact (POI).​1​
While generally spoken about in reference to CQB shooting and ARs – it’s a genuine consideration for any shooting at close distances – like those often found in bush hunting in New Zealand.
It’s also part of why 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 genuinely unique take on the dilemma. The VX-L – any guesses what the L stands for?
Basically, the lower down, the 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 pick a nominal point (100 meters or yards) and set up the firearm to align the bullet trajectory and line of sight at your chosen distance. From there, we know we can allow up (or down) when the distance to your target varies.
Your bullet will 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 hunters – and, for that matter, 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?
Don't worry - there is plenty more to read.
Well – this is where that often misunderstood and unfilled field in your ballistic calculator comes in.
Scope Height Over Bore
The term means what it says – the scope height over the bore – specifically – the middle of the optical scope tube to the middle of the rifle bore or barrel.
Let’s have a video instead of another diagram here, shall we?
I use the measuring tape method. I have run the numbers, and the difference doesn’t affect things enough for me to bother.
Talking of numbers. Let’s punch some figures into Coldbore and see what is happening. Feel free to go out and shoot the distances and see – but maths is one thing that works the same on paper and in reality. Lots of things would break if that weren’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.
Here are 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 scopes 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.
As expected, both start at the scheduled 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 up to 70 meters until the projectile is where you would be aiming.
In both cases, if the projectile is still below your line of sight/aim, what will you need to do to hit the target where you expected? Aim high. It might become clear why a lower mechanic offset for close bush hunting might be desirable. The less to offset to start with, the closer the projectile will 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 assume 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 has been based on a 100-meter zero, which 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!
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.
This is so much so that the max ordinance, where the projectile is at its highest (before zero), will be around 8.1 cm 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, are focusing on the 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 the point of aim, point of impact, at such close ranges. If you focus on one aspect while ignoring the obvious issue, the rest of the way out to 100 meters, you are waaaay higher.
The Takeaway
Like most firearms-related things, I will always encourage you to figure out the details for your specific firearm. Hopefully, this article will help explain how this works, and you can apply it to your situation. The amount of under/over will be specific to your rifle and scope setup and the calibre you are shooting. A ‘flatter’ shooting round will mean less to worry about – but – you might also be tempted to run an OPBZ – which increases it.
Learn your gear, learn how this all works. Make better decisions.