Something else you could do if you wanted to change the Unnaturals is do what it looks like FFG did in BC/OW, which is to just translate from TT.
The mechanics in TT and the RPG work differently of course, but if you look at the NPC stat blocks and compare them with TT, the various levels of Toughness Bonus appear to be correlated with the weapon stats.
By which I mean, if a weapon can hurt something in TT, it should be able to do it in the RPG.
So for example everything with a T6 or lower in TT can be hurt by an S3 weapon. Ergo, this is also the case in the RPG, which is why I assume nothing other than Daemon Engines (and Greater Daemons with Daemonic factored in, and the Leviathan from Tome of Blood) has a Toughness Bonus higher than 11 -- correlating with a laspistol doing 1d10+2 in standard mode.
In general, it seems to translate like this:
T2 = TB2 (Gretchen and such)
T3 = TB3-5 (human range)
T4 = TB6 or 8 (Orks, Space Marines -- Orks in BC/OW get True Grit and marines don't, so they are actually roughly equivalent despite Orks having a lower TB)
T5 = TB9-11
T6 = TB12
T7+ = TB13+ (undamagable by an S3/1d10+3 weapon disregarding Righteous Fury)
(Weapon damages and AP values translate similarly, with the caveat that certain qualities, mainly Flame, that increase lethality over time translate to lower immediate damage, and Tearing translates to higher damage.)
Armour values translate similarly (the one exception I can think of offhand is the Bloodthirster):
ArS 6+ = AP2-3
ArS 5+ = AP 4-5
ArS 4+ = AP6-7
ArS3+ = AP8-10
ArS2+ = AP11-14
(Weapon Pen values are correlated with this; if a TT weapon can ignore a 6+ armour save, it has a Pen of 2-3 in the RPG.)
So a Carnifex (T6 ArS3+ in TT, if I member correctly) should have TB12 AP8-10 (probably 10). Which makes it hurtable (sans RF) by both an Astartes bolter and a normal heavy bolter -- but not much.
Edited by bogi_khaosa, 05 January 2014 - 11:40 AM.