A BattleMech has eight internal locations: left, right and center torso; left and right arm; left and right leg and head (cockpit). Each location has a number of critical slots which hold components- six slots for the head and legs, twelve slots for all other locations. Components are weapons, ammunition, and equipment.
Constraints to building a 'Mech include:
Within the MechLab each 'Mech is displayed as a schematic with every location's slots available for use. For example, Figure 1 shows a new Atlas AS7-D-DC, a 100-ton Inner Sphere assault 'Mech. Stripped of all components and armor the 'Mech weighs only 10 tons due to its internal structure and fixed components (actuators, gyro, life support, sensors and cockpit). Its hardpoints are:
Figure 2 shows the same Atlas loaded for short-range combat. Now weighing its full 100 tons, it carries:
In addition, it has ammunition for the AC/20 and SRM/6s, double heat sinks and a standard engine 350. The SRM/6s have the Artemis option. To carry all this the player upgraded its structure to endo-steel, gaining tonnage but losing slots.
Certain Clan 'Mechs use "Omnimech" technology. This allows one 'Mech variant to use the locations (arm, leg, side torso or head) of another 'Mech variant of the same chassis. This allows a 'Mech to change its hardpoints.
How to Destroy a 'Mech
There are two ways to destroy a BattleMech: Destroy both legs or destroy the engine.
To destroy both legs you must remove the armor and internal structure from both legs. After destruction additional leg damage does not transfer from one leg to the other leg; additional leg damage transfers to the front side torso.
To destroy a 'Mech engine at least three slots must be lost. Removing the center torso destroys all engines since they occupy five slots in the center torso. IS XL engines occupy three slots in each side torso, so they are destroyed with the loss of a single side torso. IS light fusion engines and Clan XL engines both occupy two side torso slots, so they are destroyed with the loss of both side torsos.