Marksman scopes are an optical attachment in Operation Flashpoint: Red River, available for all three in-game factions. The marksman scope's abbreviation used in the names of the guns is "M" (so an M4A1 with a marksman scope would be an "M4A1 M").
The USMC marksman scope is based off of the Trijicon ACOG scope. In real life, the ACOG series of scopes are a series of advanced lightweight, long eye-relief scopes, usually 3x or 4x magnification. In the game, the ACOG does not provide any magnification through the scope, however, it "zooms in" more than the other sights when aiming down the sights. The ACOG has black crosshairs with small drop lines at 300, 400 and 500 meters.
The ACOG marksman scope is a good all-around sight, as no scope magnification allows the player to see around the scope, though the border of the scope itself is much thicker than other sights. The drop lines and high magnification allow longer-range shots to be reasonably accurate, without affecting close-quarters capability too much.
The ETIM marksman scope, like the PLA marksman scope, differs from the USMC model in that it functions like a low-power sniper scope rather than a marksman scope, obscuring the edges of the screen.
Like the ETIM version, the PLA marksman scope obscures the edges of the screen.
The PLA do field a sniper scope, found on the QBU88 sniper rifle, and functions similarly to the PLA marksman scope, but with higher magnification.