How to make Sniper Scope with Range pointer Digit as Sniper 3d assassian Game


when I zoom this digit is change and I fire target with this digit without cross hair
how to make this effect?
please help me
this picture IS From Sniper 3d Assassin game
first picture is zoom in 8x and second picture is 15x digit is change .

Please give me idea for this sniper digit effect
I have real bullet fire and scope but not have this digit
so I can not aim to target exactly.