Say a barghest is attacking a materialized earth spirit. Both are dual-natured, and the barghest has the Natural Weapon power for a 5P bite attack.

If the barghest bites the spirit, is the attack considered a magical weapon that bypasses Immunity to Normal Weapons? The list of what can bypass Immunity is "weapon foci, spells, adept or critter powers", and the attack is technically a critter power, but it seems like a purely mundane one.

What if the barghest is muzzled and physically attacks the spirit anyway with its standard 2S unarmed attack? Does that bypass the immunity just because the barghest is dual-natured? I would guess not.

Can the barghest attack the spirit in astral combat? Per the astral combat rules on p.184 of the BBB, "dual natured characters use their Physical attributes and skills to fight opponents with a physical body", which sounds like no actual astral combat occurs between two creatures with physical forms, even if they're both dual-natured, but it's not explicit on that count.

natural weapon being a critter power would, officially at least, bypass immunity to normal weapons.

personally, i would probably limit it's effectiveness to dual natured or astral forms, however, and would rule that a dual natured being can bypass a critter's immunity to normal weapons.

By the letter of the rules, I can see 2 rulings - 1) You rule that Immunity to Normal Weapons as not being affected by those rs unless stated in the description of those abilities/items or 2) As long as the ability/item is a weapon foci, spells, adept or critter power, it goes.