I am currently in the process of purchasing a PCP .25 cal air rifle... (Hatsan Flash QE .25 cal) I know next to nothing about rifles, scopes, etc... For an attachable bi-pod, how long does it need to be to shoot comfortably from a sitting position? Any recommendations or any to avoid? Looking at a Bushnell AR 3-9 x 40 for the scope (the 2-7 was not available).... I am more concerned about the optics from 10-30 yards than from 50-100 yards. Anything I need to know about scope rings?