Originally Posted by Mojotex

Scope swapping as you described sounds like a plan to me. I would suggest that you sight your RUM dead on at 200 rather than 300 yards. That should be plenty adequate for managing drop at 400-500 yards, and still leave you with an easy POI management at 100-200-300 yards. Check the ballistic charts for info ... better yet learn what the trajectory is of your set up testing at the range.

You might want to consider a mil-dot type reticule or one of the scope models that has an easy elevation adjustment for known distances. However, at a $600 budget, you might be somewhat pushed to find a really good low light performing 12X scope.