Ok, glad to know I'm not missing something! Yeah without many options for negating armor it takes around 22-24 damage to injure a marine I think, which is the absolute limit for a bolter
Then it seems you might have misunderstood something - any damage that exceeds armour will increase the injury level on that location, but you need to beat the target's Base Injury Value (BIV) in order to cause more than one level with a hit.
With the standard Astartes profiles though, this means that very few weapons are capable of causing a really major injury in one hit, even a short range meltagun, so for this reason I mostly recommend swapping to the Dark Magenta rules (which you already seem aware of) - these considerably lower their strength and toughness, but trade them a lot of special rules and ability to partially or fully negate certain injury effects, and it generally works a lot better.
(there doesn't really seem to be an "armor penetration" stat for weapons)
There isn't by default (the armour penetration mechanics usually halve armour) but that's arguably a good thing.
Dark Heresy (a spiritual successor to Inquisitor in many ways) has a simple Penetration stat for weapons, which ignores that many points of armour), but over the many years I've been playing in a DH campaign, it's become clear what this mostly does is make light armour pretty redundant; it's easy to get a weapon with Pen 2 or Pen 3, which makes light armour mostly dead-weight that most weapons will ignore (particularly if the GM has to keep giving enemies high penetration weapons to deal with more heavily armoured members of the party).
(Our eventual solution to this in our game was when the characters who shouldn't be walking around in half a tank worth of armour got access to forcefields, which ignore penetration).And because it's simply subtractive, and many/most enemies have some level of armour, it almost becomes extra damage at that point.
In comparison, using a dividing mechanic for armour penetration (as Inquisitor does) actually works well, because it doesn't entirely negate light armour, but remains effective against heavier armours. Inquisitor's damage system is more complex than Dark Heresy's, but it is much more nuanced.
I couldn't find it anywhere but are there crits for doubling damage or anything, like if you roll a 5 or under on a shot?
There were some optional special rules for "lucky hits", where rolling maximum damage on an attack allowed you to add another die (whichever the weapon used) worth of damage, but this is obviously quite a rare occurence for a 2D10+4 bolter. (Or the 3D8
Tearing bolters I wrote for the Revised Inquisitor Armoury, for that matter).
