Dwarf Artillery follows the Warhammer rules relating to Loss of Crew. If a war machine has been joined by a Master Engineer and/or Engineer, then they may act as crew. If so, they will suffer the consequences of any Misfire result.
In any given Shooting phase, a Master Engineer or an Engineer who has joined a war machine can either act as additional crew; use their Artillery Master or Artillery Specialist special rules; or fire personal weaponry. They must declare the option they will use before firing the war machine.
Building an army? Create, validate and share your list with New Recruit, army builder for Warhammer Fantasy Battles (6th definitive edition) and many more games.
Try the builder