These weapons can punch through even the thickest armour with ease.
When you make one or more Macro-weapon shots:
Specify whether you will target Infantry units or Vehicle units in the target detachment. 
For each shot, roll a D6. Each 4+ that you roll is a hit, regardless of armour or cover.
Allocate the hits as you would for regular Firepower, but only among units of the type that you specified.
The Space Marine Land Raider is both heavily armoured and heavily armed. It has armour-busting lascannons, which make short work of thick armour.