Rifle Scopes Ring Height/Scope Clearance CORRECT Equation

[Heronion]

I am increasingly feeling the need to correct this issue. There are several postings that give the following equation for determining if your scope will clear the barrel. They are of the form:

[front of base height] + [ring height (to bottom of tube)] - [bell diameter]/2

with a positive result meaning the scope will clear and a negative result meaning the scope will NOT clear.

This is INCORRECT. You are not factoring in the tube diameter.

Suppose your base is 0.5 inches in height. Suppose your ring height is 0.5 inches. Now suppose your bell diameter is 2.1 inches and the scope has a 1" tube. The original equation would give you a negative value, and you would say the scope will not fit. IT WILL. You have to factor in the tube diameter. By not factoring it in, you are basically saying that the centre point of the outer bell starts at the BOTTOM of the tube, which is clearly incorrect. (If you have scope caps of any kind you will have to include the increase in bell diameter they cause in the [bell diameter] part of the equation.)

The correct equation is:

[front of base height] + [ring height (base to bottom of tube, i.e. ring)] + [tube diameter with caps]/2 - [bell diameter]/2

If THAT equation gives you a positive value, the scope will fit. If negative, it will not. NOTE: ring manufacturers should say what the "height" of the ring is--this is the "ring height" that I have in the equation. The only work involved on your part is simple addition/subtraction and (if the tube is in millimeters) some unit conversion (1 inch = 25.4 cm).

Hope that all made sense to you.

Cheers,

Heronion

 

[NoExpert]

Re: Ring Height/Scope Clearance CORRECT Equation

And then if you have a rifle with any type of rear sight you have to include the rear sight height to the eye piece. I have an Excel spread sheet that factors in everything for me. Then I can reverse it by entering the ring height in another area and it'll tell me the clearance I'll have based on the manufacturer's measurements. It's also setup for rail to tube center and rail to tube bottom depending on what specs the manufacturer gives... Since I own an M14 variant I set it up for that since it's a nightmare to guess what height you may need for those rifles. However it'll work on any rifle.