Kevin Haight, P.E.
Let’s talk about reticle alignment: what it is and why it may be important to you and your precision shooting goals. Reticle alignment is the orientation between the vertical crosshair of the reticle and the vertical plane that runs through the centerline of the rifle chamber. If an optic is properly installed, the vertical line of the reticle will overlay the vertical plane which runs through the centerline of the barrel. This gets more important as the distance to the target increases, as any misalignment will result in horizontal point-of-impact errors.
Ok, let’s be a little more specific about the last sentence. Distance of shot is not the driving factor in why reticle misalignment causes issues, the amount the bullet drops is the problem. The farther the bullet drops, the greater the opportunity for a misalignment to cause an issue. And yes, misalignment will affect the horizontal point of impact…meaning the windage adjustment may be incorrect.
Allow me to get sidetracked a bit and talk through bullet drop for those that may not know, or simply want a quick reminder. This is a short article to talk about the basics, as there are entire books out that discuss the nuance of projectile paths.
What is this bullet drop?
As soon as a bullet leaves the end of a barrel, gravity starts pulling it down. The path it takes as it falls is the shape of an ellipse. Why an ellipse? The short and simple answer is gravity is constant, it is always pointing towards the center of the earth. But the speed of the bullet is not constant; it slows down as it travels, which allows gravity more time to act over a given distance. For instance, the time it takes for a bullet to travel between yard 999 and 1000 may be twice as long as yard 0 to 1. Gravity would have twice as long, over the same 1-yard distance, to pull the bullet down. Figure 1 is the basic shape a bullet would take if you held your rifle perfectly level while shooting. The horizontal line is perfectly level and the red line is the path of the bullet.
This isn’t a very practical way to aim and shoot. If it was, we would have to climb on a friend’s shoulders to be able to shoot farther distances. In reality the rifle barrel is pitched into the air and the bullet is lobbed at the target. Like throwing a snow ball at an unsuspecting friend. When we do this the projectile path looks more like the red line in Figure 2. The vertical change in elevation is still defined as bullet drop, but as you can see it might make more sense calling it “bullet rise”.
Since we are already this far down the path on the explanation, let’s add one last item to the diagram to help round it out. Figure 3 adds the dashed optic sight line and how it intersects the red bullet path. The left side of the diagram is the shooter’s location. You can see in the magnified window that the optic line starts above the bullet path. At 100 yards it crosses for the first time, which is the distance many shooters set as their zero point. 100 yards is a convenient number for many, but there are reasons you may want to “zero” at different yardages (that’s a different article). For the general shooter this would require aiming low between 0 and 99 yards, on target at 100 yards, and high beyond 100 yards.
Precision shooters will cringe if they are told to just “aim high beyond 100 yards”. Especially if the target is closer to 1000 yards. That is where the beauty of having an easily adjustable optic comes in. It allows adjustment of the optic/barrel angle, raising or lowering the bullet path to meet the optic sight line exactly at the target. Gravity is constant and therefore the bullet shape, and path, will be consistent. To go farther, lob it higher. The higher you lob it, the farther it will travel, and the more bullet drop you will have.
The beginning of the article talked about how reticle misalignment and bullet drop will result in a horizontal point-of-impact error. You should now understand the basics of bullet drop, so let’s explore why a reticle misalignment will affect the horizontal impact(windage).
Figure 4 shows the difference between a vertically aligned reticle and one that is canted to the side. Remember the bullet will always drop vertical (with gravity). This isn’t Hollywood with curving bullets, we live with Newton’s Laws where an object in motion stays in motion unless an outside force acts upon it. Those outside forces could be many things for a bullet. Wind, humidity, & temperature to name a few. To keep the discussion simple, assume that once the bullet leaves the barrel it will drop vertically, due to gravity.
The goal is to hit what you are aiming at, or, to make the bullet path and optic sight line connect exactly at the target. To accomplish this, it is critical to adjust your optic along the same vertical plane as the bullet path. If the optic is canted (by poor installation or improper leveling of the rifle) it will move the sight line off the vertical plane of the bullet path and create a windage error. (as seen in Figure 4.)
As a test to see how good you are at spotting a scope reticle that is canted, below are four reticles…which one do you think is vertical…go ahead, take your time.
Which one is it? The answer is! None of them are perfectly vertical. Ok, maybe that wasn’t fair, but the point is they range from 0.5° to 2° off and it just isn’t easy to see. Add that to a standard bubble level, which can vary between 0.5° and 0.75° depending on which line the bubble is touching, and just like that, 1° to 1.5° off!
How much is it really off?
Hopefully your next question is “how big of a windage error?” With a little bit of trigonometry, we can definitely figure it out. I love math (I know it is a sickness), so for those of you who don’t, I used the tangent function to create the chart below. Along the top are the degrees of misalignment, down the left side is the amount of drop, and the in body are the number of inches of windage error that will be created. Your specific rifle, bullet, load, and shooting distance will determine the drop. For instance, a .308 Winchester, 168 grain bullet can drop around 450 inches in 1000 yards. If you have 1° of misalignment (scope or rifle cant) it is easy to be off approximately 8” in 1000 yards.
So, what can you do about it?
Couple of things are relatively easy.
- Spend the time and money to get the rifle and optics setup as perfect as you can. A well equipped home bench will have a scope leveling tool such as the Scope Setter available through Catalyst Arms. If this is beyond what you are comfortable with, get a friend to help, or take it to a shop. Good equipment, properly setup, will always make your shoot more enjoyable.
- Use a bubble level on your rifle. Pick a quality unit, make sure it is leveled with your rifle and scope, and make sure you can see it clearly. Tip on using a bubble level: if it is a clamp on version, consider setting it such that the bubble is touching one of the side lines when the rifle is level. Consistency is the key, so it may be easier to align the bubble against one of the lines than try to guess for the center. It may eliminate a half degree of error.
- The last and least favorite is to simply ignore it. If you are punching paper at 500 yards with a 6mm Creedmore, there just isn’t going to be a lot of drop. The combination of shooter and wind will give you more windage error than a simple cant. Maybe focusing on proper shooting technique is all you need, and worry about misalignment more when you see a consistent problem.
I will leave it up to you to decide how big of a deal reticle alignment is for you. Consistency is king in precision shooting, if you want to hit the target the first time, then your rifle setup and shooting habits are critical. Happy shooting!