The ACOG Scope stands for Advanced Combat Optical Gunsight.
You can unlock the ACOG Scope for most SMG's or LMG's after 150 kills on multiplayer in Homefront. An ACOG Scope is very useful when equiped on a gun with high range, high damage and high accuracy. The ACOG is the best scope to use if you want pin-point persicion and accuracy. The downside of it is that it is quite large and obstructs the player's peripheral vision. It also have the highest amount of zoom compared to other optical attachments which makes it harder to use in close range.