[warhorse11h]

RAW indicate that if a magic item does not exist for a particular thrown spell, then an immunity to said spell is not possible. This is found on pg 156, in the section on Enchanting Objects with Immunity Spells. In the existing rules there is no entry to describe a Break Weapon magic item. By RAW it is not possible to create a magic item with an immunity to Break Weapon spells or results.

At least two distinct rules, one a talent, Master Armourer, and a second a spell, Break Weapon, both indicate the existence of said Break Weapon Immunity.

The Break Weapon spell contains a line of text that is capable of more than one interpretation. "Does not work on enchanted swords, shields, etc., constructed with Immunity (q.v.) against this spell." This text could simply be a reference to a spell effect to be described elsewhere and nothing more, in which case it does nothing to provide a solution. I submit it could also be stating that enchanted swords, etc are, by virtue of their enchantment are granted immunity to break weapons as a side effect.

I believe the latter is what the text intended to say. Note that if that is the true meaning of the text then there is no need for a house rule or house rules and RAW are valid and allow for weapons to be immune to break weapons effects.

Of course, none of the above prevents anyone from seeking a different solution, to include house rules, if they so choose.