When shooting at a war machine, shots are worked our against the entire model (both crew and machine) and any hits scored are randomised between the crew and war machine.
When shooting at a war machine, it is treated exactly as any normal unit with regards to modifiers to hit. A war machine and its crew don't count as skirmishing, and so don't benefit from the -1 To Hit penalty. Some war machines may be classed as large targets (this will be noted in the war machine's specific rules).
Once hits have been established, randomise where they strike by rolling a D6. lf 1-4 is the result, roll to 'wound' the machine. If the result is a 5 or 6, roll to wound a crewman.
Once all crew are slain, if they flee, or once the machine is destroyed, further hits do not need to be randomised but will strike a crewman or the machine as appropriate.