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Air Gun Forums » Air Guns Technical » The Air Guns Library » Effect of Scope Cant

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Old 07-10-2020, 02:00 AM   #1
dv8eod
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Join Date: Jan 2017
Location: SC
Posts: 12
Default Effect of Scope Cant

The Effect of Scope Cant
Author unknown
I have a way here, for an estimated 30% of us, to reduce group size immediately. All you need is a small spirit level, a few feet of masonry rope and a brick. If you're still with me, allow me to expound.

Put the gun on a rest. With the help of the spirit level, make sure the action of the gun itself is absolutely level. (No, not the scope, but the action of the gun. Let's ignore the scope for a minute; we'll get to that later.)

Now, at 10 yards or thereabouts (keeping in mind the focusing distance of your scope), hang the rope with the brick tied to the bottom. You've now improvised a lead (or plumb bob) - so if the wind remains below gale force, the rope will be vertical after waiting for it to stop moving, of course.

Now, with the action being level, line up the gun with the rope so that the rope coincides with the vertical line of the reticule. Check if your reticule lines up with the rope.

Now, check if your gun's action is STILL level. If it is, we can skip the next two paragraphs.

If the reticule and rope are NOT in line, it's time to loosen the mounts and rotate the scope so that its vertical reticule lines up exactly with the rope while the gun's action is precisely level.

Now, bit by bit, tighten the mounts, while checking that the vertical crosshair remains vertical (lined up with the rope with the action level).

When that's done, you're set. Now attach the spirit level to your gun (we DID ask you to bring a small one, right?) in such a way that it enables you to check whether your gun remains level as you've crouch into your favorite shooting position.

"Yeah, right! What do you think I am... mental? I'm not gonna stick a spirit level to my gun!" Well, you don't have to. Just as long as you keep in mind that there are few shots you're likely to miss. Here's why:

When you're on uneven ground, or facing an uneven horizon, you may well have canted the gun to the left or the right without being aware of it. Yes, it's that simple. Yet, the effect can be startling, especially if you're from Norfolk - or, for that matter, from the Netherlands (where almost the entire country is flatter than the proverbial pancake, and perfectly level) and you suddenly find yourself in the woodlands surrounding uneven Curridge.

This canting effect can be as much as 15 degrees without you being aware of it. If you'd actually had a spirit level on the gun, you would then notice that in order to get the spirit level to be level, you'd have to hold the gun at a seemingly awkward angle, and folks from Norfolk or the Netherlands would find themselves likely to cast doubt on that spirit level. Please, don't ask me how I know this. However, as the laws of gravity haven't had a tendency to go haywire since, say, Isaac Newton, chances are that our spirit level is right.

Why is this important anyway? Here's why. If you cant your gun without knowing, you'll still use the crosshair to aim the gun on the target. If you're using multiple aim-points or holdover, you'll use your vertical crosshair to line up with the target. If you're using your elevation turret to adjust for pellet drop, your crosshair (barring any other issues) will move in line with the vertical crosshair. Gravity, however, couldn't care less about at what angle your gun is - it will pull the pellet straight down. Look at this drawing to see what I mean:
Click image for larger version

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As you can't - wait, let's avoid possible misunderstandings.

As you cannot see on the drawing, we've canted the gun 12.5 degrees. The dotted line is our imaginary mason's rope with the brick at the bottom, but you don't often come across these on FT courses or hunting ranges.
So... we don't know that the gun is not level, so we think that the pellet will strike on the second dot under the crosshair intersection.

What reality sees from its point of view (reality knows that we haven't got the gun level), is this:
Click image for larger version

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The pellet won't drop along the vertical crosshair, but straight down to Mother Earth (along the dotted line), and lo and behold: your buddy will jot down a zero.

The above illustration is a pronounced example, but not unrealistic. Many shooters cant their guns as much as 10 or15 degrees without realizing it. On top of that, even less pronounced cant angles will, when combined with other errors, add up to dramatic alterations of Point Of Impact.
For more information and a hands on visual, visit the sight below
http://www.eatel.net/~stentorian/tar...rajectory3.swf
provided by Perry Babin and suggested by Steve-in-NC
Thanks everybody

Charlie

Last edited by dv8eod; 07-10-2020 at 07:22 PM.. Reason: Tried correcting and adding images
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