It's very likely that these will be in the Imperial Supply book, but we really don't know. I don't think Greg has even began to write that yet so it's mearly notes somewhere at FFG.
Until then Ive come up with this:
3x usual ammo cost. Add up the armour bonus and natural armour bonus, half, this is your new bonus. (Magical force effects like shield, mage armour and ring of protection are not halfed.)
i was tinking about maybe doing something to DR, but Im not sure...