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Start with a good quality base mount and mounting rings to ensure that your scope will be held sturdy for years to come. It's often better to spend a little more on the mount than on the scope itself, so you won't have to continually adjust things when you've got it sighted in. To install the mount, you'll tighten the screws into the mounts according to the manufacturer directions. It's best to proceed in an X pattern, so that you are not pulling the scope one way or the other. Start the screws loosely at first, so you can make adjustments if necessary. Lay the scope into the bracket, according to the manufacturer's directions, with the eyepiece oriented in the correct direction. Use a small torpedo level, laying it on the top of your scope to check for level, and tightening the rings firmly. Set the distance to your shooting eye so the position is correct when in your shooting position. Your image should be sharp and clear when you look through the eyepiece, when this is achieved your know you are in the correct place. Your rifle should be held in a steady position, mounted on a stand so that the rifle stock is level and square to the ground level. Start rotating the cross-hair so that the vertical cross-hair is at top dead center, or 12 o'clock. Imagine a line through the cross-hair that runs directly through the center of your rifle. You can make further adjustments later, but it's important to get the cross-hairs oriented in the correct position now, before you tighten it down. Make sure that the cross-hair remains top dead center, then gradually tighten your mounting rings. Tighten each screw only half a thread at a time constantly checking the cross-hair hasn't moved. Go slowly.
Mount the base and rings on the scope. Mount the scope on the rifle. Position the eyepiece correctly. Level your cross-hair. Tighten your mounting base securely.