My buddy would like to trade me his almost new PST 4-16 sfp and rings for my vari-x III 6.5-20x40 er/t with target dot. First of all this is a really good trade for me correct? My scope is old but perfect shape with rings, shade, and caps. He wants it for a bench gun, I want something for my ar. Second, what magnification is the mrad mil-dots correct at?