The Marksman Sight is an attachment for the Recurve Bow in Far Cry 3. The Recurve Bow is the only weapon that can equip a Marksman Sight, that is similar to the Red Dot Sight, but more advanced. There are 10, 50, and 100 meter marks that assist with shooting at far away targets.
The Marksman Sight can also be equipped on the P416, the MKG, and the ACE. However, when mounted on these weapons, it looks to be based on a Trijicon ACOG.
In demos of the game, the Marksman Sight appears to be available for the MP5.