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What Do the Numbers on a Rifle Scope Mean?
In a 3-9x40 scope, the 3 means three power, or 3x. This means that the image you see through the scope appears three times (3x) closer than it does with your naked eye. The 9 means nine power, or nine times (9x) closer than it appears with your naked eye. The forty (40) is the objective lens diameter in millimeters. This is a variable scope because you can vary the magnification of the scope from three to nine, stopping anywhere in between. You would describe this scope as a "three to nine by forty."
Some scopes have a 3x magnification range such as a 3-9. Some have 4 or more as in a 4-12. Some new scopes have a magnification range of 8x or more. The larger the magnification range, the more you will pay, but it makes for a more versatile scope. For hunting and most applications, a 3-9x40 scope will be one of your best options. 3x power is perfect for taking closer shots, and the 9x power is more than enough to accurately acquire elk or deer-sized game at longer ranges.
Most scopes, especially in America, have main tubes that are one inch in diameter. That means that they use one-inch rings. Some scopes have thirty-millimeter main tubes. Those scopes will use thirty-millimeter rings. There are several main types of bases that are used to connect the rings to your firearm. You need to know what kind of base you have to find out the exact type of the one-inch or thirty-millimeter rings you will use for your specific scope. The height of the rings is determined mainly by the objective lens diameter, but also the barrel thickness, action size and type, ocular bell diameter, and bolt lift. Here is my article on Choosing Mounts, Rings, and bases.
