Field of view is the biggest factor - 1-6x46 or something like that range is where you are going to be looking. The biggest field of view you can get which is usually at a very low magnification... The reason for this is at close range, single digit meters anything bigger than 3x is a toilet tube and you might struggle to resolve what you are looking at (the difference between seeing a part of a head and visible eyeball and ear with naked eyes, and then through the scope having to work out what each individual hair or fur bit is attached too).
A secondary factor is actually fitting the thing to your rifle, as some of the lower mag scopes are shorter in body length which won't get to the right place for your eye with some mount setups.
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