A Cannon or a Stone Thrower (but not a Flame Cannon) which has been joined by an Engineer may make two guesses when declaring the range they are firing at (one for the crew, one for the Engineer). Measure the two points you have guessed and then decide which one you want to use for that shot before rolling any dice.

A Bolt Thrower which has been joined by an Engineer may use his BS when rolling to hit.

Note that if the Engineer uses his ability during the firing of the war machine, he will not be able to shoot with his own missile weapon in the same Shooting phase (he is too busy grumbling about the crew's poor aim). Remember that you must fire all the weapons requiring you to guess the range before any normal shooting. In addition, you must remember to fire a Bolt Thrower which an Engineer has joined before he opens fire with his own weapon. If you fired the Engineer's own weapon then you cannot use his re-roll ability in the same turn.

When firing his own missile weapon, an Engineer can always choose a different target from the one fired upon by the machine he has joined.