Rifle Scopes : New To Distance Shooting

I PM'd 338LM about his 3-15x42 SWFA SS scope - even though I probably won't NEED the higher magnifications.

Like I said, this rifle will be used to hit garbage lids at 500 yards and under, not trying to smash any golf balls at this time.



I've been looking at these scope rings: http://www.competitor.com/egw-30-mm-tactical-aluminum-scope-mount-rings.html

With a 42mm objective, do I need low, medium, or high? and are those good rings?

Also, where can I get a picatinny base for the mossberg patriot?