quad gun hit by multimelta

if the quad gun is shot with a mulitmelta and wounded does it get its 3+ save? or is it instant killed?

As the multi melta is AP1 then no, the quad gun does not get its 3+ save. It may be eligible for a 4++ cover save depending on where the shot comes from as the defence line or bastion may provide cover.

As it is T7 it will not be subject to instant death

It doesn't get a 3+ save because the melta is AP1, if it's behind the defence line it gets a cover save tough. However the quad gun is T7 and the metla is S8 so it only loses a wound (it has tow wounds). You need the Strength of a weapon be twice that of the targets toughness value to instant kill it.

Ninjaed!