Most scopes are all numbered pretty much the same, but do you know what those numbers mean? Do you know what 3 to 9 by 40 millimeter means? The first two numbers, 3 to 9 equate to the magnification. I have 3 power magnification. I have a variable power scope, so I can turn the power selector ring from 3 power to 9 and everywhere in between, which is great, because if I'm shooting in dense, thick woods and I'm only shooting 25 or 50 yards, 3 power, wide field of view. Don't need to bring that animal in close. 9 power, "Hey you know what? I'm shooting 100 yards, 150 yards. I've got a good solid shooting rest. I can make that shot, so I need to bring that target in close."
What's the 40 millimeter mean? That equates to the objective lens. 40 millimeter's pretty standard. It gives you good late transmission through the scope. Now you also hear of a 50 millimeter scope. Bigger objective lens, which again allows more light to come through the scope. More light means you could shoot later in the evening.
One area that's commonly overlooked in scope selection is how big is the ocular lens of your scope? Ocular lens, being the lens by your eye. The larger the ocular lens, the more head room you have when you mount the gun. You can move your head around and place it directly down the bore of your rifle. It's just easier for head placement.
When you make a scope selection. Think about the range of power, magnification, certainly think about your objective lens size. Whether you think you're going to be needing that extra five minutes of light. Then also, look at the size of the ocular lens. This one here's pretty standard, but look at the size of the ocular lens on my rifle scope. It's a lot larger, so therefore head placement on my scope is less critical.