3-9x40 minimum I would say. But 4-12 or 4.5-14 would/could be better as shots CAN go to 600yrds or more. Dont go crazy and get an 8-32x 60 or anything tooo large. Get the BDC or rangefinder reticle as an aid to the long range shots. I dont recommend mildots as they are too small, but many use them here.

Nikon makes a 4.5-14 with BDC that is afine scope, as are other brands.

And good luck.