If your just looking to put on a good clear scope, then the sky and your budget are the limit. If you want to compete in NRA & CMP match's then rules apply. MUCH has been written on the military competitive forums on what optics rule there.
( From recent NRA rule change)
"The rifle may have an optical sight (reflective sights are considered optical sights) with a
maximum power of 4.5X installed on the receiver. Variable scopes with a maximum of
4.5X are permitted. Only commercially manufactured scopes that were produced with a
maximum magnification of 4.5X and that have a maximum objective lens of 34 mm may
be used. If an optical sight is used, the same optical sight must be used in all stages of a
course of fire (changing sights is not permitted). The centerline of an optical sight shall be
no higher than 3.5 inches above the centerline of the bore."

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Dave