Canting - a thought

C

cdoc42

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Although not directly related to handloading, it might come into play when you are testing your experiments for accuracy.

I read a web site that reviewed the science behind canting but it was more than I could absorb, leaving me with, "I believe you." It seems more significant as distance increases but I am left with the thought that there might be some element of concern at 100 yards, which could explain the difficulty in getting all shots in the same hole.

Basically, canting is rotating the axis of the rifle to the left or right, which moves your point of impact to the left or right, respectively. The first thing that came to my mind was "How often do I make sure my targets are 100% vertical?" The answer was, I never thought about it. But if your target contains a vertical line or a diamond shape to allow you to align your vertical reticle, if the target is not completely vertical, you will be canting your rifle.

If you use a traditional rest with a rear bag and your target is nothing but a circle bullseye, chances are you will cant one or more of your shots in a series that becomes a "flyer."

How many use a level to set targets and/or to level you rifle with each shot?
 
With your scope aimed on target center, pick something near the side of field of view to align the reticle cross wire on. Repeat this same alignment for each shot and cant angle will be consistent enough to not be a problem. Or use the top, left or bottom. Makes no difference which one as long as it's the same one for each shot.

Sine of cant angle times range bullet drop equals horizontal bullet impact change.

Cosine of cant angle times range bullet drop equals vertical bullet impact change after you subtract it from bullet drop.

5 degree right cant and 20 inch drop equals 1.743 inch right and .076 inch low; relative to point of aim.

Vertical error insignificant in most applications.

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Would a 5 degree cant with a load sighted in at 200 yards produce an impact point lower at 100 yards than trajectory tables in the manuals list? That is, if the book lists a rise of 1.5 inches at 100yds with a 200 yard zero, would it be lower? I assume the POI at 300 yards would be lower and to the right as well.

Or maybe I just did a bunch of things wrong that day even if I did hold level.
 
Cosine of 5° is 0.9962. If I launch a 175-grain SMK out of a 300 WM at 2930 fps, using a telescopic site mounted 1.5 inches above the bore line and zeroed at 200 yards, POI will be 1.5 inches high at 100 yards, but if you fired it perfectly bore-sighted instead of using the adjusted sights, you would discover total drop of the bullet during the 0.1059s TOF in 1 Earth gravity is actually 2.117 inches We multiply 2.117 inches by 0.9962 and get 2.1089. Subtract that from 2.117 inches and you get 0.008 inches of drop. So, instead of impacting 1.5 inches high at 100 yards, you would impact 1.492 inches high at 100 yards.

As Bart said, you can pick anything vertical that is in your field of view for helping get your sights upright. This is true of iron sights, as well. I use the target paper (either mine or those on the adjacent targets, depending on the range) to confirm my front sight post is upright.

NOTE: While it is correct that you need consistent sight alignment to test ammunition, you need it for other kinds of shooting as well. For that reason, I suspect this really belongs in the Art of the Rifle. I will move it.
 
While certainly not the most precise method, am using the scopes vertical line. Tilt to left till obviously canted to left, tilt to right till obviously canted to right and then split the difference. Matters not how the target is hung and is fairly quick, while keeping eyes on the target. Seems to work good at 600 yds and under.

Learned this while cutting joints in concrete walk to get close to square of the curb line.
 
Erno86, that's a nice idea. The other thing I thought of today is how everyone shoulders their rifle. I mounted a scope for a friend using a leveling tool and when I picked the rifle up and aimed at a vertical wall joint, everything looked fine. When he shouldered the rifle he said the reticle was not vertical. So I changed it to match his view. Did I cant his rifle?
 
cdoc42 said:
So I changed it to match his view. Did I cant his rifle?
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Yes. It will shoot the cant direction from point of aim.

Adjust the scope back then ask him to level the reticle himself by twisting the rifle else he will have windage errors at all ranges, more as range increases. Scope may need to be rezeroed.
 
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Cdoc,

I once gave a friend's hunting rifle scope a good mechanical vertical and handed it to him and he said the vertical crosshair was still tilted a little. I told him to switch to his weak side hold. He said: "Hey! Now it tilting the other way."

Have the guy look through the sight standing, sitting, prone and on the bench. Then have him try the weak side. For most people that shifts personal vertical enough that he can see the mechanical vertical you set is a good compromise.


Ern86,

I've used both the plumb line and a self-leveling laser line (indoors) to get a scope set up. The thing I did with the plumb line outdoors is set the bob close to the ground and then when there was a lull in the breeeze, watched to identify the point it was swinging around. That point is where it would be in dead air. I turned a fat screw hook into the ground at that point, but a tent stake would work. Tie the string off to it. If you want to get fancy, hook a screen door spring onto it and stretch it a couple of inches and tie the string to its upper end. That tension will take up slack. That compensates all day for the tendency of the string to gradually stretch. The nice fluorescent orange plumb line is easiest for me to see.
 
My scope is a Bushnell Elete 6x24x40 definitely not a top of the line scope , it does have a slight can't but doesn't make a difference because I'm shooting always at 200 yards . If your going to shoot at a set distance it's not a problem , for hunting or if your lucky enough to have ranges in your area that go far , I would send the scope back , have it fixed or replaced or you will blame every missed shot on the scope. Low to the right or low to the left as you go further out. .
 
cw308,

Does the scope W and E adjustments track the reticle? If so, the scope is good. It's twisted in its rings.
 
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Bart B
The original base and rings one piece came from Iron Brigade Armory ( tactical Scope mount ) 1" rings , if I remember correctly with a built-in 20 rise for distance , that worked fine with the Nikon 14 power but when I wanted more power the new scope had very little adjustment and went with the Ken Farrell base and rings with a zero rise . Worked fine torqued everything . When using the wheeler crosshair leveling kit , even the Segway reticle leveler , my scope caps weren't in line with the crosshairs . I could see the target better and shooting at the same distance it wasn't a problem . I even use a thermal heat lable on my barrel from McMaster-Carr. I baby my rifle , I read articles on what ever I'm thinking of changing on my rifle and asking questions on this forum . When setting up a scope I'm pretty careful , when I was a teenager it was a different story , just screw it down so it don't move and be careful not to get that half moon scar through your eyebrow.
 
Scope caps do not have to align with a reticle wire. The E and W adjustment axis must be parallel with a reticle wire. A slight angle difference is okay from cap to wire.
 
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