I don’t know how many times I hear these types of statements:
- I zeroed my rifle at 25 yards, but when I shoot it at 100 yards, it is x inches high.
- I zeroed my rifle at 50 yards, but it isn’t zeroed when I shoot at 100 yards.
- I zeroed my rifle the other day at my Dad’s farm, but when I took I to the range, it wasn’t zeroed anymore.
In each case, the shooter wants to know what is wrong with their optics or their ammo. Because, well, nobody ever taught them about ballistics and offsets. Gravity is a bitch and makes life a challenge for shooters that don’t understand its impact. Hey, I understand that everyone know that gravity exists, but I don’t think they all really understand how gravity impacts their bullet.
Here are some of the major factors that will impact the zero of your rifle:
- Bullet weight
- Bullet velocity
- Ballistic coefficient
- Height from center of barrel to center of optic
Of course, there are other impacts, such as wind and other atmospheric conditions, which would include temperature, height above sea level, humidity, and so on.
For this article, I will use a round that I shoot from my 300 Blackout rifle. Why? Because I understand it and how it works.
110 grain supersonic – This bullet, a Hornady V-Max, has a ballistic coefficient of .290 and I build it so its speed at the muzzle is 2080 feet per second.
The center of my sight is 2.5” higher than the center of my barrel. It is very important to know this distance.
Yes, we really need to discuss one of the most basic forces on our lives. Remember, from those science classes where we all napped, that everything falls at the same rate when all variables are removed. The same is true of a bullet. So, if we were to drop a rock at the exact same moment we fired our round, the bullet would hit the ground at the same instance as the rock. Bullets are not magic, and can’t defeat gravity. The difference is that the rock will fall down straight, but the bullet will fall away from us because of the forces we exert on it by shooting it out of the barrel.
No matter how fast the bullet flies, it is still falling towards earth just as fast as the rock.
So how does the bullet hit that target 100 yards away? Well, simply because we are actually shooting it out at an upwards angle. The angle is incredibly small, but it is there. Since the bullet travels in an arc, it can, potentially, intersect our line of sight through our optic, two different times – once on the way up and once on the way down.
Basically, since the bullet starts out below the sight, as shown in the graphic, it must travel up to intersect our line of sight. In this case, the rifle is zeroed at 200 yards, but it also crosses our line of sight at about 50 yards.
The graphic is exaggerated to make it clearer and easier to understand. BTW, yes, this is one that I pulled from the Internet and modified a bit. I confess. I pulled it from aesirtraining.com, but I do not know if that is the original source.
Gravity Summary: Yes, the bullet must be shot at an upwards angle, otherwise it would never intersect with our line of sight.
Here is what my round looks like, when you run it through a ballistics calculator. I ran it using a 25 yard zero (in red) and a 100 yard zero (in blue).
Some of the important items to note:
- Since my sight is 2.5” over my barrel, I automatically start at –2.5” for each zero.
- When I zero at 25 yards, the angle is a bit higher for the barrel than when I zero at 100 yards. Basically, the bullet needs to go up 2.5” faster for the 25 yard zero.
- When I zero at 25 yards, it is actually zeroed at 25 yards AND at about 219 yards.
- When I zero at 100 yards, it is actually zeroed at about 68.5 yards AND at 100 yards.
The Question: Based on the two zeros shown, which would be the best for me to use?
The answer is not too difficult. You just need to know what ranges you plan on shooting from, both short and long.
For example, if I knew that everything I would be shooting would be between 25-250 yards, I might prefer a 25 yard zero as it will, at most, make my bullet a little less than 5” high and less than 3.5” low.
Alternatively, if I knew that everything I would be shooting would be between 25 yards and 150 yards, a 100 yard zero might make the most sense as it will, at most, make my bullet no more than 2.1” off target.
There are many good ballistics calculators out there. Just use your favorite search engine and look for a good ballistics calculator. One of my favorites is the one at JBM.