The Deploy Weapon special command instructs the gunner of a heavy weapon to deploy his weapon (on the appropriate mount), while one or more other soldiers of the same unit are designated as loaders and/or security or lookouts.
Most heavy weapons cannot be fired at all without being properly deployed. When they are not deployed, these weapons show on the unit information window as "not deployed" or "limbered" (for guns). A few heavy weapons may be deployed optionally; in this case deploying the weapon increases its chance of hitting or its fire output. These weapons are shown as "semi-deployed" when not deployed.
Deployment takes a specific amount of time for each type of weapon, and also depends on various other factors, such as the unit’s experience and current condition. Depending on the weapon system, certain restrictions may apply as to where a weapon can be deployed or not. For example, some weapons may not be deployed inside buildings or on balconies or roofs. Others may be deployed, but the deployment time is increased: for example, heavy machineguns may deploy inside buildings but it takes more time.
If you order a unit with a currently active Deploy Weapon command to move, it will first automatically "undeploy" the weapon, and then execute the movement command. Just as for deployment, undeployment takes a certain amount of time. If you issue the Deploy Weapon command to an already moving unit, it will deploy its weapon at the end of the move. If the move command consists of several waypoints, the weapon will be deployed after the last waypoint has been reached.
All weapons which can be deployed will be automatically deployed in the Setup Phase of a battle. Anti-tank guns that are deployed in the Setup Phase and do not move or rotate are harder for the enemy to spot!