Here's another set of rules from last game.
Shooting at Artillery, 40k7 pg 64
If shooting at an Artillery unit, the Toughness of the guns is always used whilst at lesat one gun remains.
Context: Tactical squad was shooting at Vaul Wraith Weapon Support. For some reason I got it in my head that majourity toughness means I'd be rolling to wound against the crew not the guns. Shawn corrected me.
Shooting with Walkwers, 40k7 pg 90
Range is measured from the weapon itself and line of sight is measured from the mounting point of the weapons and along its barrel, as normal for vehicles.
Context: Opponent wanted to have his war walkers shoot at tactical squad. The squad was behind the walkers. Opponent thought walkers had a 360 rotating field of fire. Informed LOS is from the weapon and walkers are facing away from the squad.