There are more methods how to measure the required height of the scope rings based on the dimensions of your rifle, and dimensions of the riflescope.
In this guide I will use the one that I find the easiest. And you can use this method even if you dont have the scope with you. You will only need the measurement details provided by the manufacturer of the riflescope.
If you dont have the scope with you
If your scope is not physically with you, you can simply check the website of the manufacturer. The measurement that you will be looking for? The overall diameter of the objective bell.
Once you know this information, divide that number by 2 and add about 2 milimeters for clearance. This number is the minimal possible height of your scope rings.
If you have the scope with you
If your scope is with you, you can get this number simply by measuring it yourself. Then you will again divide the diameter by 2 and add 2 milimeters for clearance. The number that you will get is the minimal height of your scope rings.
If you do not have anything to measure with
In the rare event of you not having anything to measure the diameter at all, you can simply estimate the number. How? Take the diameter of the objective lens [that is the last number written on the scope], and add few extra milimeters for casing. How many milimeters for the casing? It is usually between 4 and 8 milimeters overall. Classic polymer / metal casing has usually 4mm, thicker ones from rubber can go closer to the 8mm casing.
Example
If your riflescope has on the lens written 6-24x50, then the lens is 50mm.
Then you will add 4 to 8mm to the casing, and you are at the number of your bell diameter between 54 to 58mm. [If the casing is classic polymer / metal, its probably 54mm, if its thick and rubber, its probably closer to 58mm.]
To get to the minimal height of the scope rings, you just divide that number by two, add 2mm for clearance, and that is the minimal height that your scope rings need to have.
Why to add 2mm for clearance?
So that you can be sure that the scope will not be toughing the barrel.
What to do next
Once you have the minimal height of your scope rings, you will do best to look for the scope rings that are the closest to that height. Obviously, they cannot be lower than that.
You may also want to look for higher scope rings, but that is usually if you know well why you do that (for example your rifle is equipped with adjustable cheek rest, you use additional leather cheek rest, etc.)
Other things to consider when selecting scope rings
Except of the height of the scope rings you also need to consider the diameter of the main tube and the type of scope rings. We go into the debth in this article [How to select the right scope rings] but for now you can be fine with knowing that there are three most used diameters of main tube. And thus three diemeters of scope rings:
- Scope rings with 1inch diameter,
- Scope rings with 30mm diameter, and
- Scope rings with 34mm diameter.
Scope rings for picatinny rail
If you use scope rings with picatinny base, keep in mind that the picatinny base will add few milimeters to the measurement. If the picatinny rail ends before the scope bell (so that the scope bell will not be touching the picatinny rail), you can take that into consideration and lower the minimal height that you calculated by the height of picatinny base.
