The holographic sight is an upgrade to enhance ranged accuracy, akin to the Laser Sight.
It's practically the most useful attachment for a weapon, and is typically used the most out of any other attachment.
An attachment is placed onto a firearm by dragging the attachment onto the firearm the player wishes to place the attachment onto. The firearm must have an open attachment slot in order to be able to have the attachment placed onto it.
Below is a list of firearms that mods can be attached to:
Holographic sights can be obtained in three different ways: