Lots of choices out there I had gone with a 4x16 Nikon Monarch scope on my RRA varmet with a 20 inch bull barrel. I have been using the Nikons on my hunting guns and happy with them. I find the light it gathers during the last 15 min of the day helps me to get shots that I couldn't with open sites.
I have A nikon monarch UCC for sale if you are interested it's A 6.5-22x44 nikoplex reticle illuminated red and green brand new still in box I mounted on A rifle for A couple of months but never shot it good glass pm if interested.