Quoted:
If you had to choose between the small or large aperture, which would you choose?
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Under stand that the two apertures are not on the same Height distance up from the top of the receiver, and if you look at your elevation knob, will see two numbers.
Hence large aperture is the smaller of the number, and small aperture is the larger of the number.
So with elevation dial set to 3/6, large aperture will be you 300 set zero, while small aperture up, would be your set 600 zero with the front sight post untouched.
As for if you are not going to use the rear sight this way or the elevation dial, and instead just go with one or the other and zero it out alone for a given distance, then just depends that your going to use the rifle for. In most cases, such as running target that you will need to lead, it's easy to use the larger aperture, since it gives you more side view of the front post for lead distance.