Practise is definitely important but to know for sure where your bullet will strike at 300 yards you must actually shoot the gun and load at 300 yards.If you never do that you are merely guessing at your trajectory.Trajectory tables are a guide but are often not that accurate at the longer distances and even moreso if you don' t chronograph your loads to determine the exact velocity that your load produces in your gun.Velocities printed in manuals can vary a great deal from the actual velocities that your gun produces.

Smitty to answer your first question, my friend send his Leupold vari X III back to leupold twice becasue it wouldnt hold a pattern. He even bought the adjustable Leupold rings to go with it. Everytime they sent the scope back they said there was nothing wrong with it so I decided to look at it myself. What happened was the Scope was undersized by only .003 and was moving in the rings. Common practice now-a-days is to use some thin 3m double sticky back tape (its about .003 thick) and lay it on the bottom of the ring mount then install your scope and upper ring halfs. After doing this it elliminated my friends problem. I do it on all my scopes and most experienced dealers use this method too.

While leupold does make bases with windage adjustment I was not aware that they make adjustable rings.If either the rings or scope were not made to the correct dimensions leupold will gladly replace them for free.It is not normal to have to place any shim material in rings to make a scope fit.