Take it to a qualified gunsmith and have him explain the whole process to you. Each rifle must not only have accurate sights, but must be zeroed for YOU and to a specific ammo at a specific range. 20' low at 50 yds sounds pretty severely "off" for any possible combo though. You won't spend much, you'll get an accurate rifle, and you'll probably learn something. Besides, any time you buy a used gun you should have it thoroughly checked out by a trusted gunsmith before firing it.

------------------
My New Year's resolution is to have more fun...even if YOU don't.