The top hatch on the back of a Chimera may be opened to allow models inside to shoot out. This allows weapons other than lasguns to be used. Up to one model transported in the Chimera may shoot from the hatch, in addition to any models using the Chimera's lasguns. However, if they do this then the Chimera counts as being an 'open-topped' vehicle for the rest of that turn and all of the opponent's subsequent turn. The model may shoot any type of weapon, not just a lasgun, including heavy weapons that require two crew (even mortars). Models shooting from the hatch must engage the same target unit as models firing lasguns. Remember that heavy weapons may not be fired from the hatch in the same turn as the Chimera moves.
Top Hatch
Source: Codex: Armageddon, p. 29