In part 1 I spoke about riflescope eye relief, how to select a riflescope with respect to eye relief and your rifle and how to mount your riflescope, again within the context of eye relief. In this article, I will discuss scope cant, what it is, its effects on the pellet’s impact point and how to eliminate it.
what is scope cant?
Scope cant is the misalignment of the vertical reticle line of the scope with the centerline of the rifle. If the two are not aligned, you will find that whilst your pellets are on target at your chosen zero range, they will miss at further distances where holdover is required. The further the range from zero, the greater the error. The rule is that if your scope is canted to the left, your pellets will strike low and to the right of your aim point. If your scope is canted to the right, your pellets will strike low and to the left. But hang on, you say, I’ve heard that if canted left the pellet strikes left and vice versa. Well, that’s true for “rifle” cant where the scope is properly aligned to the rifle but the shooter is not holding the rifle level. I’ll talk about that later. For now, let’s focus on “scope” cant.
We all know that the cross on your reticle is the point of impact of your pellet at your zero distance. When aiming at distances further than the zero range, we add holdover in order to reach the target. As the reticle is slanted, you will unknowingly rotate the rifle to place the aim point onto the target. This rotation has the effect of changing the trajectory of the pellet from straight ahead to left to right or right to left across the centre line. This can be difficult to visualise. Imagine the barrel is attached to the side of a cone. The muzzle is fixed at the point of the cone and the breech attached to the lower rear edge of the cone. As you turn the rifle to place the aim point onto the target, the breech moves either left or right and up, whilst the muzzle remains fixed in place. This not only changes the trajectory to the left or right of the aim point but also lowers the incline of the barrel causing the pellet to strike lower on the target. There is also a slight increase in flight path due to the angle of incidence to the target which will also cause the pellet to strike lower still.
what is rifle cant?
Rifle cant is almost the same as scope cant except that both the scope and rifle are correctly aligned initially. The cant is introduced by the shooter as the rifle is shouldered. The difference for the shooter is that the pellet still strikes low but in the same direction as that of the cant. Some shooters fit bubble levels to their rifles to help eliminate this phenomenon but you could also use visual references such as the horizon, fence posts, etc whilst maintaining visual contact with your target through the scope.
To ensure your scope is correctly aligned with your rifle you will need a few basic tools. A bubble level, a plumb line, scope mount Allen keys and a stable platform to hold your rifle. I use an MTM Predator shooting rest but you could use a rifle bag rest instead. The important thing is to ensure your rifle doesn’t move.
For a plumb line, I used an old scuba diving weight block and some string. I hang it at the end of the garden from the washing line pole and let it settle. I bought the bubble levels from a local DIY shop. I do not recommend using the bubble levels that attach to your scope mounts on their own as you will need to use another level on the action of the rifle to be certain that the scope level is actually aligned to the rifle. By all means, buy one to help with rifle cant, but do ensure it is correctly aligned to the rifle using another bubble level resting on the action of the rifle. Otherwise, you will find you are actually adding cant whenever you shoulder your rifle.
Some rifles make it almost impossible to rest a bubble level on the action. For example, the HW77 and HW97 air rifles have round actions which makes the method above impossible. However, all is not lost as you can use a dovetail rail-mounted bubble level. Also, if you use one of these bubble levels, there is no need for a second level on the stock of the rifle.
Before starting any work on your rifle, ensure that it is safe. Check that it is not loaded and that it is not cocked. You should do this religiously whenever you take your rifle, or any other firearm, out of its case or bag regardless of whether you think it is already safe. Complacency leads to accidents and accidents can injure or worse, result in death.
aligning the scope
I’ve assumed that you have already set the scope up for the correct eye relief. This needs to be done first otherwise all your efforts to align the scope will be wasted.
Find a place that is as long as the minimum focus range of your scope where you are legally allowed to use your rifle. Pointing your rifle down the street from indoors really isn’t a good idea either! Setup the plumb line downrange. Perhaps on a tree branch or something where it can hang freely to ensure it is vertical. At the other end, maybe the shooting line of the range, set up your rifle on the gun rest. Set the focus of your scope so that you can clearly see the plumb line.
Using your scope clamp Allen key, loosen the tube clamps just enough so that you can rotate the scope. Place a bubble level on the action of the rifle if there is a flat place to put one. The action would be the preferred place to use a bubble level, but if not, you could use the stock or scope mount if they are flat. However, these secondary locations can’t be guaranteed to be level with the action but they’re going to be as close as you can get. Now adjust the rifle on the gun rest so that the bubble is centred in the level. Your rifle is now vertically aligned to the plumb line.
Look through the scope whilst being careful not to move either the rifle or the scope. Rotate the scope so that the vertical line of the reticle is aligned with the plumb line. You may need to adjust the direction of the rifle to place the reticle over the plumb line. Recheck the rifle is level and readjust both the rifle and scope as necessary until you are satisfied that the two are aligned with the plumb line. Now tighten up the scope clamps and perform a final check to ensure that the scope has not rotated.
Knowing that your scope is correctly aligned you can confidently move ahead to the next stage of setting up your rifle by zeroing the scope.
Now that you have a little understanding of scope and rifle cant, it’s worth making a conscious effort to ensure your rifle is not canted when taking aim. Maybe you could fit a scope bubble level or use visual references to level the reticle whilst looking through the scope. Part 3 of this series of articles will explain the basics of Chairgun Pro, a free software tool from Hawk Optics, how to select your optimum zero range and then calibrate your scope mil-dots to coincide with 5-yard increments.
Until next time, happy shooting!