Dope for your scope!

Mike Morton offers a few practical tips to help newcomers work out exactly where to aim their rifle – regardless of the distance to the target.

Couldn’t it be great if we could zero a rifle, then just pick it up and shoot it at different targets and at different distances, always hitting the mark?

Sadly, until hand-held laser guns become a reality, or unless we shoot our airguns in a zero-gravity environment like space, that’s just not going to happen. Instead, regardless of the type of airgun we’re shooting or the calibre of pellet we’re using, our projectiles will always be affected by gravity.

Unless we intend to shoot our guns only at one specific distance, as you would with one of the target disciplines such as ISSF 10 Metre Air Rifle, we need to understand what effect gravity has on our shooting, and how to compensate for it so we know where to aim to hit a chosen target at whatever range it presents itself. This means creating a diagram or table showing us where to aim at a variety of distances.

Long-range centrefire shooters call this a DOPE chart, standing for Data On Previous Engagements, but this is a vital concept for airgun shooters too. We need to work out where our shots will fall so we can then work out how to adjust our aim.

If our shots are falling lower than our point of aim, we need to aim higher to compensate. This is called holdover. Conversely, if our shots are hitting higher than our point of aim, we need to aim lower, and this is known as holdunder.

Before we get into the nitty gritty of working out holdover and holdunder, let’s explore why gravity is so important, even though we’re shooting over relatively short distances.

A pellet leaving the muzzle of an air rifle will immediately begin to trace a downward arc, but that arc isn’t constant; it becomes steeper the further the pellet travels.

This is because as soon as that pellet is fired it will start to slow down, and the slower it becomes, the longer it’s exposed to the effects of gravity. In practical terms, this means the pellet will drop more, the
further it travels.

There’s a misconception that when a pellet is fired it initially rises before eventually dropping. That’s not really true, it’s just that the barrel is  angled up in relation to our line of sight through the optic, giving us the impression that the pellet is rising.

If we were to take a shot with the barrel perfectly level, then the pellet would always hit lower than where the barrel is pointing. Instead, we need to point the barrel higher than our target and then let gravity pull the pellet down so our point of aim coincides with the pellet’s point of impact.

Luckily, gravity is fairly easy to account for. We just need to create an accurate DOPE chart or card to show us how our pellets drop at the ranges at which we intend to shoot, then work out if and when we need to holdover or holdunder, and by how much.

Ballistic Calculator?

It’s possible to use one of the many ballistic calculators that are available on the internet, and these are usually free to use. While most are designed for centrefire or rimfire shooters, there’s no reason why they can’t be used for airguns too.

Keep your DOPE chart and reticle aimpoints to hand such as in your range box, in your pocket, or better yet – actually on your rifle

You will have to input some very specific data, however, such as muzzle velocity, the weight of the pellet, its ballistic coefficient (this can usually be found online), the distance at which the rifle’s been zeroed and your sight height – the height of the centreline of the scope relative to the centreline of the bore.

Getting good results from a ballistics program relies on inputting peerless data, so you’ll need to use a chronograph to find the figures you need. It’s also important to verify your findings by shooting your rifle and correlating those results to your reticle.

In many ways it’s simpler and more time-efficient to just get out there and shoot, carefully recording your results as you go. So that’s what we’ll do here.

What’s the aim?

If you’ve never carried out this process before, it’s worth pointing out that the creation of a DOPE chart will only work for one rifle at one power output that’s been zeroed at one distance with one type of pellet of a particular weight using one particular scope.

And in the case of a second focal plane scope, this chart will only work at one particular magnification setting too. In other words, it’s extremely specific.

If you change any of these variables, then you will have to start again. So before taking the time and trouble to do this, make sure you are happy with your chosen gun, scope and pellet combination.

What you want to achieve is twofold: create a copy of your reticle with the specific aimpoints marked in for specific distances, and measure the difference in point of impact from point of aim when using the same aimpoint at all distances – you’ll see why a bit later.

Let’s get practical

While you won’t need a chronograph to help you calculate your holdover and holdunder points, you will need to find an online image of your scope’s reticle and be able to download it and print it off. If you don’t have access to a printer, you can just sketch an image of the reticle, paying particular attention to the elevation stadia.

I created my own DOPE data using my BSA R-10 SE in .22 calibre, shooting the Rangemaster Sovereign, a JSB-made pellet. The rifle is fitted with a Hawke Sidewinder 30 SF.

This is a second focal plane scope which is equipped with Hawke’s 10x ½ Mil Dot IR reticle and has a magnification range of 4-16x. I went to the Hawke Optics website (www.hawkeoptics.com), found an image of the reticle and printed it off. It’s important to have this to hand when moving onto the shooting phase.

The BSA R-10 is a regulated PCP, and I made sure it was filled to the correct standard working pressure before beginning. The next step was to verify the rifle, scope and pellet combination was correctly zeroed at my chosen zero distance, which is my standard 30 yards.

But before doing this, it was time to make an important decision. Because this scope is a second focal plane model, I had to choose which specific magnification I wanted to use to work out my aimpoints. I decided to go with the full 16x.

The object of the exercise is to shoot at targets set out at different distances, always aiming at the centre of the target, then making a note of the fall of shot relative to the markings on the reticle.

This is a hunting rifle which is primarily shot prone off a bipod, so I wanted to gather DOPE data from 10 yards out to 40 yards, my self-imposed maximum hunting range with this rifle, at five-yard increments. In my case, that meant I would have to shoot a total of seven separate targets. Had this been an HFT gun, I’d have shot a ninth target at eight yards and a 10th target at 45 yards as well.

It’s important to treat the creation of a DOPE chart like a pellet test; we’re trying to find and record accurate data, not test our marksmanship or wind estimation skills, so the rifle should be shot from as stable a position as possible in wind-free conditions, otherwise your results may be skewed.

My target of choice is a self-adhesive 1” Target Spot made by Birchwod Casey, but any target will do – even a black dot drawn on a blank piece of card with a marker pen.

It’s also a good idea to write the range at which they’re being shot on each of the targets, and this will give you the orientation too – you’ll need to know which way is up when interpreting your results to know whether some shots went higher or lower than the bull.

When shooting at these different distances, you can move yourself closer or further away to the target to alter the range, but if you’ve established a stable firing point and if the conditions allow, it’s usually easier to move the target instead.

I like to shoot a few pellets at my chosen zero range to begin with, confirming the rifle is properly zeroed, taking note of any wind and (most importantly) making sure I’m alert and in full-on shooting mode.

In this case, my rifle is zeroed at 30 yards, but it has what is known as a secondary zero at 10 yards. This means the pellet is being fired gently upwards, intersecting my line of sight at 10 yards, flying above the line of sight between 15 and 25, falling back on the primary zero at 30 yards and then falling below line of sight at 35 and 40 yards.

I now know I need to aim directly at the target at 10 yards and 30 yards, aim below it at 15, 20 and 25 yards, and aim above it at 35 and 40 yards. And because I marked up my reticle, I also know exactly where to aim at each distance – as long as I keep my magnification at 16x.

Changing Magnification

This only applies to second focal plane scopes, but if you want to use a different magnification setting, you can either repeat the process and record a second set of aimpoints, or use a bit of maths.

If I were to halve my magnification to 8x, I would only need to holdunder or holdover by half the spacings I worked out for 16x. Either way, it’s still important to verify your calculations by actually shooting your rifle using these new aimpoints.

Sprayed adhesive or double-sided sticky tape can be used to tack your aimpoint reticle to the inside of your scope

One thing that will always be constant, regardless of your magnification setting, is how far above or below the bull your shot landed, so you can also use this as a handy guide to work out where to aim as long as you know the range at which you’re shooting as well as the size of the target.

You can now transfer your raw findings onto a neat DOPE chart to record your fall of shot, and a copy of your reticle accurately marked up to show you where to aim. If you resize the reticle using Microsoft Word or a simple online tool such as ImResizer (https://imresizer.com) you can even make one to fit inside your scope cap, so these aimpoints are always to hand.

The only thing left to do is get out there and shoot! 

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