‘Cant’ shoot straight?

As if range-estimation, trajectory, crosswinds and our ability to hold the rifle steady aren’t enough challenges to overcome in order for our pellets to hit the mark, there’s also another factor, which many shooters don’t even realise comes in to play. Cant – or the leaning of the rifle, either to the left or right, from the vertical plane.

In the past, I’ve spoken to many an airgunner who tells me it’s too insignificant a problem to worry about. These shooters often cite the fact that most 10-metre paper target shooters lean the rifles to extraordinary degrees – and I won’t disagree with that last fact. However, I will disagree with the first – for those of us who don’t shoot match guns, canting the rifle is the perfect recipe for missing.

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You see, in match shooting, the shooter aims his air rifle at a target that’s a fixed distance away – usually 10 metres. Providing he mounts his rifle the same each time, it doesn’t really matter if he’s shooting with the rifle upside down. However, if you’re shooting at targets that could be at variable distances, then the phenomenon of cant becomes a very big issue. In short, we don’t want it.

To understand why canting a rifle is an issue for both hunters and FT or HFT competition shooters downrange, you first need to understand the relationship between the scope – and, therefore, its crosshair – in relation to the rifle and its barrel.

601-2_scope levellingIdeally, the scope should be positioned in the perfect vertical plane as the rifle. To achieve this requires a very methodical scope-fitting procedure using a plumb line – and although it’s a little long-winded, it’s very important not to rush things. If you don’t get the scope/rifle relationship right in the first place, you’ll never be able to effectively eliminate cant – because if the rifle’s ‘upright’, the scope won’t be… and vice versa.

The figures below illustrate a scope which has been incorrectly mounted. In this case, it’s likely that when the shooter brings the gun to his eye, he will cant the rifle to compensate for the skewed crosshairs – so that once he’s happy the reticle is perfectly horizontal and vertical, the rifle will in fact be ‘leaning’.

Figures 1 2 2a LAYERS3 Figures 1 2 2a LAYERS2 Figures 1 2 2a LAYERS

 

 

 

 

 

 

 

 

Having understood the gun/sight relationship, it then becomes a little easier to understand the cant-induced problems downrange. When the rifle is fired, the pellet falls to earth. In practise, the barrel is pointed slightly upwards (which is why the pellet’s flightpath crosses the sightline at two points, one about 10m away and the other at the ‘zero’ distance).

However, regardless of the barrel’s angle of elevation, the pellet travels downrange on the same vertical plane as the rifle – sometimes higher up the crosshair (ranges nearer than zero) and sometimes lower down it (distances further than zero). But it will always be in the same plane as the scope’s vertical crosshair.

However, when the rifle is canted  the pellet is not going to be travelling along that same vertical plane as the scope’s vertical crosshair. The crosshair may look perfectly aligned, but because the gun’s actually leaning, the pellet is going to travel in a completely different plane (the yellow dotted line) than that of vertical crosshair (the red dotted line). Of course, you wouldn’t zero a rifle up to miss to one side all the time.

So when you zeroed the rifle, you would have dialled in windage adjustments to compensate for the lateral deviation of the pellet. Unfortunately, that will be the only point on the pellet’s trajectory that will coincide with the vertical wire of the crosshair. At distances nearer or further, the pellet will be travelling in a plane that’s either to the left or right of it, according to which way you’re leaning the rifle.

400 target groups

This scenario has ramifications when you’re giving either hold-over or hold-under. Say you’ve zeroed your rifle at 25 yards and you take on a target 35 yards away. Ordinarily you’d aim a little higher. But if you’re canting the rifle, the longer shot is also going to strike to either the left or right. This reduces your chances of landing a perfectly central shot on target, even if you’ve gauged the correct amount of elevation.

You can prove this to yourself quite easily. The target in figure 4 shows what happened when I took aim at a mark (in this case a pellet hole) on a 35-yard target with a rifle that was zeroed at 30 yards. Group A shows the fi ve-shot group with the rifle held correctly – slightly below my aimpoint, as you’d expect. Group B shows the cluster when I deliberately canted the rifle clockwise, and group C when I canted it anti-clockwise. As you can see, the ‘drop’ is roughly the same (as you’d expect), but the group has shifted laterally by an amount that’s actually more than the drop itself.

7102_SP3 7101_SP1It’s also noticeable that the cant-induced groups aren’t as tight because the amount I leaned the rifle wasn’t a consistent angle. In the last illustration, my scope/rifle combo had been accurately aligned in the first place using the plumb-line method, so you can see that errors will still occur if you inadvertently cant the rifle. Therefore, it’s vital that you do everything in your power to bring the rifle onto a non-canted aim – and there are some accessories that can help you in this respect (see panel opposite). I’d recommend that you invest in them for the very reason that alleviating cant isn’t always easy in the field.

This is especially true if you shoot in undulating terrain, or frequently take steeply-elevated shots. Your brain is a very powerful ‘computer’, but even it can be fooled by an optical illusion – and once you’ve fitted a ‘bubble’ to your rifle, you’ll be amazed at how often your brain fails you in the field. If you’re shooting on a hillside, for instance, what might look ‘vertical’ to you when you peer down the scope could very well be out by up to 15 degrees or so when you look at the bubble of any spirit-level device you may have fitted.

And the same thing applies if you’re shooting high up into the treetops – even on flat land. As soon as you go past around 40 degrees, it becomes extremely difficult for your brain to work out what is a ‘true’ vertical and horizontal plane, with the result that you’ll often be canting your rifle to an extraordinary level without even realising. Refer back to those groups in figure 5 and you’ll soon see why having some form of level-checking device on your rifle is an absolute must.

We’ve all had those inexplicable misses and more often than not, they can be put down to cant – either because you’ve unwittingly leaned the rifle over, or because you never aligned your scope correctly in the first place. If you ever have one of those days when you say to yourself you simply ‘can’t’ shoot straight… well, you’re right. ‘Cant’ is the operative word.

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Posted in Features, Target Shooting

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