How much firespower is needed for a weapon to be able to perform a breaching attempt? You need at least 4 to be able to destroy a building but the rules dont say anything about breaching except that it needs to be a heavy weapon.
As it’s not stated go with the literal interpretation, Thomas, so a Heavy weapon with explosive effects, whatever it’s FP. As it says, walls are easier to breach than demolishing a building.
It look as if most suitable Heavy explosive effect weapons listed are 3 or 4AP: eg SMAW 4AP,2AT(H); AT4 3AP,3AT(H).
As the Building Strength is -1D when a breach attempt is made that may help a 3FP weapon such as the AT4 and if successful units within are hit with a +1FP to the weapon’s FP too.