Scopes don't change the maximum effective range of firearms, that is a function of the velocity and aerodynamics of the round. Just a quick look at the weapons table, and the real world knowledge I have of the weapons, tells me that realism was not concern when designing weapon ranges however.
Example: 5.56 Assault Rifle in the D20 Modern book has a range increment of 80 feet. This is a max range of 800'. The real maximum range of an M16A1 is about 460 meters. The more recent A2 is closer to 550 meters. The farthest target on the standard Army qualification range is 300 meters, still farther than the listed maximum for the book.
Now if you add in Far Shot the ranges come a little closer together. Close enough I think to preserve the semblance of reality (grammar Nazi's read verisimilitude).