To me, the most important elements of an air rifle hunting scope in no particular order are…
1) clear, quality glass; this should be a given
2) dependability and durability; again, this should be a given…holds zero like a rock, tracks well, consistent Point of Aim through magnification/parallax adjustment
3) Second Focal Plane; when it get's dark or the game is close and you need low magnification or there is a dark and varied background…all three of which are normal hunting experiences for me, at least; you don't want a first focal plane scope for most hunting scenarios
4) low magnification/wide field of view; there is nothing more frustrating and aggravating than trying to shoot a squirrel that is too close and you can't find it in your scope; my favorite hunting scopes are 1X or 2X on the low end for this reason…3X or 4X are still acceptable
5) parallax adjustment; when shooting at close distances it's very annoying when the parallax/focus is out of wack and it definitely affects accuracy; something that I've found is that low magnification scopes struggle far less with this issue than higher magnification scopes; for example, my Leupold VX-II 1-4x20mm is focused and clear down to about 10' at 1X and 25' at 4X…but is still very clear and focused at all further distances…which brings me to my last element…
6) simplicity; give me a standard or maybe even thicker than average duplex reticle or comparable reticle that is not too busy or distracting; I'm not saying that there is never a use for BDC's or mil hashes/dots or Christmas trees…but in hunting fast acquisition and centering the quarry is paramount; KISS
7) mount it low; this is the least important to me but I think it still is important for several reasons…I hesitate to buy a scope with more than a 44mm objective
This an old post By B.B. Pelletier on pyramydair.com. I find it works perfectly for me out to about 30-35 yards for a dead hold…and it works better if your scope is mounted as low as possible.
"For a pellet gun that shoots around 800 f.p.s., I like to sight in at 20 yards for the near distance. The second distance will be around 30 yards, and the pellet will not rise by as much as one pellet diameter at the in-between distances (between 20 and 30 yards). If you sight in at 15 yards with the same gun, the pellet will be back to the intersection of the crosshairs around 40 yards, and it will rise more than an inch in between.
For a gun that shoots 950 f.p.s., I would still sight-in at 20 yards as the near distance but the far distance is now 37 yards or so. For both guns (800 and 950), the pellet will be about one inch below the aim point at 10 yards and will rise to the crosshairs as it approaches 20 yards. "