This post deals with one of the most over looked or unknown factors of accuracy for the average shooter or sportsman. It is called "Cant". My intent is to help those who don't know about it and refresh some memories for those who may have forgot about it.
Introduction
In shooting there are many factors that affect the ability of a shooter to hit a target with accuracy. One of these factors is cant error. Cant error is the result of not holding the rifle bore axis and the scope axis in a vertical plane.
The Problem
To begin to understand cant error, let's review some basic information, starting with gravity. As we all know, gravity is a very important part of our lives and we continually compensate for it. For example, when we are outside tossing a football around we throw it with only a slight trajectory for a short pass, or for the long bomb we add a lot of 'elevation' to the trajectory. We use this same compensation in precision shooting. If we are shooting an air gun with a slow flying projectile, we use a lot of elevation to hit the target. Gravity acts on the vertical component of the projectile and its effect is proportional to the time of flight. From physics we know that when two objects of dissimilar weight are dropped from the same height, at the same time, they will hit the ground at the same time, this of course disregards wind resistance of the objects. If a high power rifle and a pellet gun are fired at the exact same moment parallel to the ground, the two projectiles will fall to earth at roughly the same rate (remember wind resistance) and will strike the ground at the same time. It's just that the high power bullet will have traveled many times the distance that the pellet traveled because of its higher velocity. As a result, the elevation compensation that is required for a low velocity air gun pellet may be many times greater than what is needed for a high velocity rifle, but both need elevation compensation to counteract gravity. The elevation compensation required to hit a distant target is handled within the sighting device on the rifle. When a scope is mounted on a rifle it is almost always parallel to the bore of the firearm. Figure 1 shows the trajectory of a typical long-range application where the projectile trajectory (solid line) passes through the line of sight (dash line). In the figure, a slight angle between the scope and the rifle barrel is depicted. In reality, the scope and rifle are mounted parallel to each other as discussed above, and this angle is created within the scope optics which adjust the elevation. The elevation correction within a scope points the line of sight downward, which in turn points the bore axis up when the sight is aligned with the target.
The representation in figure 1 enables one to visualize the line of sight axis in relationship to the bore axis. Because your eye is the point of reference, when a rifle is unintentionally canted the line of sight becomes your axis of rotation. Cant error is generated when the barrel axis rotates out of the vertical plane (Plane 1 in figure 2), around the line of sight axis in either a clockwise or a counter-clockwise rotation. The height your scope is mounted above your bore affects the amount of scope cant and aiming cant errors. The following figures illustrate how cant will affect the impact of the projectile.
![Image]()
Figure 2, shows an isometric view of Figure 1 with a target at the far end. The relationship between the scope and the barrel does not change when we cant the rifle. Figure 2 shows the cant hold which is rotated 20° counter-clockwise from the vertical hold. This results in the cant trajectory, which shares the line of sight with the vertical hold but the path of the projectile alters from the vertical hold. The cant error shows up at the impact point. The cant error is due to gravity and barrel rotation. The impact point in this case is low and left of the center of the target.
![Image]()
Figure 3, shows an end view of Figure 2. Notice how the trajectory of the vertical hold stays within the vertical plane, so when the projectile drops, it drops into the line of sight and down to the center of the target. The trajectory of the cant hold does not achieve the same height as the trajectory of the vertical hold and the projectile diverges from the line of sight, thereby missing the target. When a rifle is canted, the included angle between the bore and the line of sight, (that compensated for the effects of gravity in the vertical hold), is rotated and acts as a windage error directing the projectile's trajectory laterally off the desired course. This component of cant error becomes more significant at more distant targets due to the increased original included angle between the line of sight axis and the bore axis (more elevation compensation) at the vertical hold. The canted projectile's trajectory does not achieve the full height at the apex of the flight. This accounts for the elevation error at the point of impact. Gravity is still the same, so the drop of the vertical path D1 is equal to the drop in the canted path D2. Use of large diameter objective scopes, mounted high off the barrel, exacerbates the cant error problem. This type of scope is mounted high off the barrel to clear the large diameter of the objective lens. To keep the scope elevation knobs centered for maximum adjustment, precision shooters sometimes use elevation compensated scope mounting rings or bases. Although, this solves the adjustment problem, it greatly exaggerates cant error because the distance between the bore axis and the line of sight axis increases and the included angle between the sight axis and the bore is larger, producing more windage error when canting. This still does not explain all the error we see in the test results. To see why, we need another view of the cant error figure.
![Image]()
Figure 4 shows a top view of the cant versus vertical hold trajectories with respect to the line of sight axis. The vertical hold trajectory and the line of sight axis are coincident. The cant hold trajectory in this case veers left and continues to diverge further left. The "error triangle" (depicted in Figure 2), constructed at the apex of the trajectories (shown here as happening at the same point in time for simplicity) describes the entire cant error at the target. The error triangle incorrectly predicts the windage error as the base of the right triangle and the elevation error as the height of the right triangle. In Figure 4 we show that the cant hold trajectory continues to diverge after the apex of the trajectory is reached because the bore of the rifle is actually pointed slightly left and once started on that diverging path, the projectile will continue on that path throughout the length of the flight. Since the velocity and distance of the canted shot is identical to that of the vertical shot, the total drop of each shot is identical. However since the canted shot never reaches the full elevation (lower apex) of the vertical trajectory it will drop below the vertical shot impact point. One can imagine that with extreme long range shots the projectile travels under its induced windage error path for long distances creating large errors. A similar exaggerated windage error effect will be seen for slow projectiles which require larger compensation angles between the line of sight axis and the bore axis. This larger included angle will induce larger windage errors when canted.
Solution
Elevation adjustments are made to compensate for the gravitational pull on the projectile to the earth's surface, which is in a vertical plane. By inadvertently rotating the rifle around the line of sight axis we are inducing a cant error into the shot. Some people believe they can hold the rifle in a vertical position but when you are out in the field there is no true reference for being horizontal to the horizon. Most people tend to use the horizon or the ground to estimate the position. This usually leads to unintentionally canting the rifle and inducing some amount of error, which at long ranges can be very significant. Anti-cant bubbles are difficult at best to try and line up and hold while concentrating on the target. They are distracting to the shooter who needs his full attention on the target. Even very small deviations from a vertical hold can affect the impact point of the projectile, especially at long ranges or with slow projectiles. The Microlevel® is an excellent solution to eliminating cant error while not distracting the shooter from his target.
Now that we all know what "cant" is and before we move on to the test below lets watch this video on mounting a scope. It will show you the proper way to mount it, how to mount it level and it will talk about adjusting the scope for a clear reticle for proper parallax adjustment.
This is an easy test to determine if your reticle is perpendicular to the bore and requires less ammunition than shooting the BOX. But before the test lets zero our rifle at 100 yds. Here is a good video for sighting in your scope.
Now that were zerod at 100 yds. Staple a fresh sheet of cardboard or butcher's paper to the target backer. Using a 4 foot contractors level scribe a plumb line on our paper with a "Sharpie felt pen". Make this line 20-30 inches long and at the bottom of the line, scribe a horizontal line that will act as our aiming reference. For those of you who are artistically inclined we've created an inverted capital "T".
![Image]()
From the bench, align the reticle's vertical wire with the plumb line on the paper and aim at the intersection of our lines. Fire a single round and note its location, It should be dead on at the intersection of the vertical and horizontal lines that you drew. Dial 15-20 MOA of "UP" elevation in our scope and aim at the same reference line. Be sure the reticle is aligned with our plumb line on the paper and fire a second round. If all is well, our bullet should be within a bullet diameter of striking our line, indicating our reticle is indeed perpendicular to the bore. We have now confirmed that our elevation knob is tracking true to the center line of our bore. Any extreme elevation correction will now launch the bullet inline with our intended target given a dead calm condition.
![Image]()
If for some reason the shooting Gods have frowned upon you and the bullet strikes 1-2 inches "left or right" of the line, a scope rotation is in order. We must first note the location of our scope to the ring cap, take the Sharpie pen and make a "WITNESS" mark on the scope tube and ring. We next loosen the ring cap screws and "rotate" the scope in the direction of our bullet impact. A slight rotation is all that is necessary, be precise in our rotation, a 1/32 or 1/16 rotation will make a big difference in point of impact. Re-zero the rifle ( yes, this is an important step and must not be overlooked) and repeat the above exercise until the bullet strikes our vertical line.** Don't forget to paste the old bullet holes!
You've now successfully confirmed that your reticle is perpendicular to your rifles bore. Slip the windage/ elevation knobs and set them to "0". Do NOT turn the knobs to set to "0" you must loosen the knobs and set them to "0" then re-tighten them. Grab the bino's, Data Card and start searching for a distant target to shoot. You'll be confident that reticle perpendicularity is no longer a concern when you need to stretch the barrel on your next long shot!
Now that your scope is mounted correctly, squared to the bore and zerod perfectly let's watch this next video on the fundementals of shooting.
If we take the time to be confident in our equipment by periodic testing and evaluating. We will eliminate our "equipment related" shooting slumps and can isolate areas that require our focused attention. Position, sight alignment, breathing, trigger squeeze and follow thru are area's we can ALL improve upon.
All too often shooters blame errant shots on equipment, ammunition, imperfect zero's and a host of other excuses. Often times we find that by retracting our eye from the ocular lens we can see the reflection of who is indeed responsible for the missed shot? You will never make any real progress in your shooting if you do not take responsibility for your actions...
Now that we have the Cant issue resolved and our rifle is sited in perpendicular to the bore let's talk about Parallax, or better yet, lets watch the video below. The only thing i disagree with in this video is where he states if your parallax is set to 100 yards and your eye is off center and your shooting at something 400 yards away your impact will be 2 inches off. I disagree, you will be way off and probably miss your target all together. other than that he does a great job explaining parallax, check it out.
I hope the information in this post helps some people.
Bottom line, eliminate all the factors you can before you hit the field.
Thanks for taking the time to read this.
Introduction
In shooting there are many factors that affect the ability of a shooter to hit a target with accuracy. One of these factors is cant error. Cant error is the result of not holding the rifle bore axis and the scope axis in a vertical plane.
The Problem
To begin to understand cant error, let's review some basic information, starting with gravity. As we all know, gravity is a very important part of our lives and we continually compensate for it. For example, when we are outside tossing a football around we throw it with only a slight trajectory for a short pass, or for the long bomb we add a lot of 'elevation' to the trajectory. We use this same compensation in precision shooting. If we are shooting an air gun with a slow flying projectile, we use a lot of elevation to hit the target. Gravity acts on the vertical component of the projectile and its effect is proportional to the time of flight. From physics we know that when two objects of dissimilar weight are dropped from the same height, at the same time, they will hit the ground at the same time, this of course disregards wind resistance of the objects. If a high power rifle and a pellet gun are fired at the exact same moment parallel to the ground, the two projectiles will fall to earth at roughly the same rate (remember wind resistance) and will strike the ground at the same time. It's just that the high power bullet will have traveled many times the distance that the pellet traveled because of its higher velocity. As a result, the elevation compensation that is required for a low velocity air gun pellet may be many times greater than what is needed for a high velocity rifle, but both need elevation compensation to counteract gravity. The elevation compensation required to hit a distant target is handled within the sighting device on the rifle. When a scope is mounted on a rifle it is almost always parallel to the bore of the firearm. Figure 1 shows the trajectory of a typical long-range application where the projectile trajectory (solid line) passes through the line of sight (dash line). In the figure, a slight angle between the scope and the rifle barrel is depicted. In reality, the scope and rifle are mounted parallel to each other as discussed above, and this angle is created within the scope optics which adjust the elevation. The elevation correction within a scope points the line of sight downward, which in turn points the bore axis up when the sight is aligned with the target.
The representation in figure 1 enables one to visualize the line of sight axis in relationship to the bore axis. Because your eye is the point of reference, when a rifle is unintentionally canted the line of sight becomes your axis of rotation. Cant error is generated when the barrel axis rotates out of the vertical plane (Plane 1 in figure 2), around the line of sight axis in either a clockwise or a counter-clockwise rotation. The height your scope is mounted above your bore affects the amount of scope cant and aiming cant errors. The following figures illustrate how cant will affect the impact of the projectile.
Figure 2, shows an isometric view of Figure 1 with a target at the far end. The relationship between the scope and the barrel does not change when we cant the rifle. Figure 2 shows the cant hold which is rotated 20° counter-clockwise from the vertical hold. This results in the cant trajectory, which shares the line of sight with the vertical hold but the path of the projectile alters from the vertical hold. The cant error shows up at the impact point. The cant error is due to gravity and barrel rotation. The impact point in this case is low and left of the center of the target.
Figure 3, shows an end view of Figure 2. Notice how the trajectory of the vertical hold stays within the vertical plane, so when the projectile drops, it drops into the line of sight and down to the center of the target. The trajectory of the cant hold does not achieve the same height as the trajectory of the vertical hold and the projectile diverges from the line of sight, thereby missing the target. When a rifle is canted, the included angle between the bore and the line of sight, (that compensated for the effects of gravity in the vertical hold), is rotated and acts as a windage error directing the projectile's trajectory laterally off the desired course. This component of cant error becomes more significant at more distant targets due to the increased original included angle between the line of sight axis and the bore axis (more elevation compensation) at the vertical hold. The canted projectile's trajectory does not achieve the full height at the apex of the flight. This accounts for the elevation error at the point of impact. Gravity is still the same, so the drop of the vertical path D1 is equal to the drop in the canted path D2. Use of large diameter objective scopes, mounted high off the barrel, exacerbates the cant error problem. This type of scope is mounted high off the barrel to clear the large diameter of the objective lens. To keep the scope elevation knobs centered for maximum adjustment, precision shooters sometimes use elevation compensated scope mounting rings or bases. Although, this solves the adjustment problem, it greatly exaggerates cant error because the distance between the bore axis and the line of sight axis increases and the included angle between the sight axis and the bore is larger, producing more windage error when canting. This still does not explain all the error we see in the test results. To see why, we need another view of the cant error figure.
Figure 4 shows a top view of the cant versus vertical hold trajectories with respect to the line of sight axis. The vertical hold trajectory and the line of sight axis are coincident. The cant hold trajectory in this case veers left and continues to diverge further left. The "error triangle" (depicted in Figure 2), constructed at the apex of the trajectories (shown here as happening at the same point in time for simplicity) describes the entire cant error at the target. The error triangle incorrectly predicts the windage error as the base of the right triangle and the elevation error as the height of the right triangle. In Figure 4 we show that the cant hold trajectory continues to diverge after the apex of the trajectory is reached because the bore of the rifle is actually pointed slightly left and once started on that diverging path, the projectile will continue on that path throughout the length of the flight. Since the velocity and distance of the canted shot is identical to that of the vertical shot, the total drop of each shot is identical. However since the canted shot never reaches the full elevation (lower apex) of the vertical trajectory it will drop below the vertical shot impact point. One can imagine that with extreme long range shots the projectile travels under its induced windage error path for long distances creating large errors. A similar exaggerated windage error effect will be seen for slow projectiles which require larger compensation angles between the line of sight axis and the bore axis. This larger included angle will induce larger windage errors when canted.
Solution
Elevation adjustments are made to compensate for the gravitational pull on the projectile to the earth's surface, which is in a vertical plane. By inadvertently rotating the rifle around the line of sight axis we are inducing a cant error into the shot. Some people believe they can hold the rifle in a vertical position but when you are out in the field there is no true reference for being horizontal to the horizon. Most people tend to use the horizon or the ground to estimate the position. This usually leads to unintentionally canting the rifle and inducing some amount of error, which at long ranges can be very significant. Anti-cant bubbles are difficult at best to try and line up and hold while concentrating on the target. They are distracting to the shooter who needs his full attention on the target. Even very small deviations from a vertical hold can affect the impact point of the projectile, especially at long ranges or with slow projectiles. The Microlevel® is an excellent solution to eliminating cant error while not distracting the shooter from his target.
Now that we all know what "cant" is and before we move on to the test below lets watch this video on mounting a scope. It will show you the proper way to mount it, how to mount it level and it will talk about adjusting the scope for a clear reticle for proper parallax adjustment.
This is an easy test to determine if your reticle is perpendicular to the bore and requires less ammunition than shooting the BOX. But before the test lets zero our rifle at 100 yds. Here is a good video for sighting in your scope.
Now that were zerod at 100 yds. Staple a fresh sheet of cardboard or butcher's paper to the target backer. Using a 4 foot contractors level scribe a plumb line on our paper with a "Sharpie felt pen". Make this line 20-30 inches long and at the bottom of the line, scribe a horizontal line that will act as our aiming reference. For those of you who are artistically inclined we've created an inverted capital "T".
From the bench, align the reticle's vertical wire with the plumb line on the paper and aim at the intersection of our lines. Fire a single round and note its location, It should be dead on at the intersection of the vertical and horizontal lines that you drew. Dial 15-20 MOA of "UP" elevation in our scope and aim at the same reference line. Be sure the reticle is aligned with our plumb line on the paper and fire a second round. If all is well, our bullet should be within a bullet diameter of striking our line, indicating our reticle is indeed perpendicular to the bore. We have now confirmed that our elevation knob is tracking true to the center line of our bore. Any extreme elevation correction will now launch the bullet inline with our intended target given a dead calm condition.
If for some reason the shooting Gods have frowned upon you and the bullet strikes 1-2 inches "left or right" of the line, a scope rotation is in order. We must first note the location of our scope to the ring cap, take the Sharpie pen and make a "WITNESS" mark on the scope tube and ring. We next loosen the ring cap screws and "rotate" the scope in the direction of our bullet impact. A slight rotation is all that is necessary, be precise in our rotation, a 1/32 or 1/16 rotation will make a big difference in point of impact. Re-zero the rifle ( yes, this is an important step and must not be overlooked) and repeat the above exercise until the bullet strikes our vertical line.** Don't forget to paste the old bullet holes!
You've now successfully confirmed that your reticle is perpendicular to your rifles bore. Slip the windage/ elevation knobs and set them to "0". Do NOT turn the knobs to set to "0" you must loosen the knobs and set them to "0" then re-tighten them. Grab the bino's, Data Card and start searching for a distant target to shoot. You'll be confident that reticle perpendicularity is no longer a concern when you need to stretch the barrel on your next long shot!
Now that your scope is mounted correctly, squared to the bore and zerod perfectly let's watch this next video on the fundementals of shooting.
If we take the time to be confident in our equipment by periodic testing and evaluating. We will eliminate our "equipment related" shooting slumps and can isolate areas that require our focused attention. Position, sight alignment, breathing, trigger squeeze and follow thru are area's we can ALL improve upon.
All too often shooters blame errant shots on equipment, ammunition, imperfect zero's and a host of other excuses. Often times we find that by retracting our eye from the ocular lens we can see the reflection of who is indeed responsible for the missed shot? You will never make any real progress in your shooting if you do not take responsibility for your actions...
Now that we have the Cant issue resolved and our rifle is sited in perpendicular to the bore let's talk about Parallax, or better yet, lets watch the video below. The only thing i disagree with in this video is where he states if your parallax is set to 100 yards and your eye is off center and your shooting at something 400 yards away your impact will be 2 inches off. I disagree, you will be way off and probably miss your target all together. other than that he does a great job explaining parallax, check it out.
I hope the information in this post helps some people.
Bottom line, eliminate all the factors you can before you hit the field.
Thanks for taking the time to read this.
