Beam
(Rule)Description: When firing with a weapon with the Beam trait, the controlling player draws an imaginary straight line 1mm wide from the weapon, or the firing model if the weapon is not physically represented, up to its maximum range; if the weapon has an Arc, the end of the line must be within the specific Arc of the firing model. Make Hit rolls for each Detachment (friend or foe) that has at least one model under that line equal to the Dice value of the weapon - models with the Flyer special rule are ignored and not hit. Any model from each Detachment you make Hit rolls for that is within range of the firing model and within the correct Arc (if applicable) can be allocated scored Hits.
If the line would pass over an area of Impassable terrain or a Structure then the line 'stops' as soon as it hits them - any Detachment further out does not suffer Hits, though Detachments Garrisoned in the Structure do.
Beam weapons cannot be fired as part of an Overwatch. As a weapon with the Beam trait does not choose a dedicated target, the Detachment may pick any eligible target to fire on, including a target that is not hit by the Beam.