Rifle Scopes and Mounts
What the Scope Numbers Mean
The numbers for each scope tell you the scope’s power or magnification rating, whether the magnification can be increased or decreased with a zoom dial, and the diameter of the objective lens.
So, a 3-9x40mm is a scope with a variable zoom magnification from 3x to 9x with a 40 millimeter objective lens.
How Much Magnification do I need?:
Because high-powered riflescopes have a smaller field-of-view than a lower powered scope, choose your magnification with the distance you will be shooting in mind. Use low power for close shots in brush or deep woods and high power when hunting distant game in the mountains, open fields.
Objective Lens = The front lens of the scope
AO = Adjustable Objective - The ability to be able to focus over different distances
MilDot = Military dot
Scope ring mount Information
The section of the mounts is a very important process.
There are a number of different types of dove tails that the scopes rings can mount on, so it is important that you select the correct type to suits your rifle, check with the manufacture on what dovetail type you have.
Most Air rifles and rimfire rifles have 9.5 – 11.5mm dovetails. BSA Rifles are about 13mm, where as Tikka and most CZ rifles are 15mm, but be aware some CZ rifles can be 17mm which also suits Parker Hale.
Then angle of the 9mm to 17mm dovetails follow the same angle. Weaver and Picatinny use a different angle and are some times wider again.
These days the Objective lens size can be larger that what the original mounts could handle.
With this in mind you should select the correct Height that is required to allow your scope to fit your rifle with the minimal clearence between the bottom of the scope and the rifle barrel.
They can come in the following height types. Low, Medium, High, Extra High and Ultra High. You would have to check with each scope ring manufacturer on what height they call Low-Ultra High
To calculate the Ring Height, you can use the following formula.
H = (Objective lens size / 2) - (Scope tube diameter / 2)
e.g. For a Scope that has a 55mm Objective and a 1" (25mm) tube.
H = 55/2 - 25/2
H = 15mm