The DShK is first seen being used by the Cuban guards of Castro against Alex Mason and his team in the level, "Operation 40". It is frequently seen in "Vorkuta", mounted on the back of trucks manned by prison guards. After jumping from his motorcycle to one of these trucks, Mason uses a DShK to fire at enemy motorcycles, trucks, and Mi-8 helicopters. During "Executive Order", the DShK is seen mounted on trucks, but never usable. During "WMD", Hudson and his team use a similar truck, with Hudson using the mounted DShK to provide cover while the truck is started.
It also appears in emplaced form, and can be manned by the player in the campaign levels "Iron Lady" and "Down the Rabbit Hole". In "Down the Rabbit Hole", it can be used to hold back against the Inner Circle before evac will arrive in any minute.
As for Spec Ops, players in Juggernaut suits will be attacked by this weapon mounted on a vehicle turret in Toxic Paradise, and the weapon can be found and used as an emplacement in the DLC mission Negotiator.
A DShk is seen in "Traffic", right before breaching a room to rescue Samuel Abidoyo. It's also used by an enemy on the top of a truck in "Fission".
The DShK in "Traffic"
The sights on this weapon are unique compared to other mounted machine guns featured in the Call of Duty series; the sight is a circle with the crosshair formed by a wire strung through a chain. It also has an unusable deflection-finding AA sight on the left side of the gun which uses three wires running horizontally through its center, spaced close to each other.
When firing, the ammunition belt does not move.
On the Special Ops mission Negotiator in Modern Warfare 3, the player's hands do not appear on the weapon.