Damage as a function of HeroPower
Lets start over again with some science (third? fourth time now?), after some testing it seems like the damage we do is linear to HeroPower, and that we have a base “Power” of 180. Below I present a table with scaling modifiers with different bases; 0, 20, 100 and 120.
Important
I have data for two different weapons, one in the interval [15-217] and another in [225-517], both are linear in respective intervals, but I cannot prove that all weapons are linear across the entire spectrum. And note, everything above 517 HeroPower is just using the same linear model.
How to read the table:
All numbers are factors of the damage caused at “base”, for example the damage factor at 100 HeroPower compared to HeroPower 15 is 1.44 (see column “Dmg base 15” and go down to row “100”).
| HeroPower | RawPower | Dmg base 0 | Dmg base 15 | Dmg base 20 | Dmg base 100 | Dmg base 120 |
|---|---|---|---|---|---|---|
| -180 | 0 | 0.00 | 0.00 | 0.00 | 0.00 | 0.00 |
| 0 | 180 | 1.00 | 0.92 | 0.90 | 0.64 | 0.60 |
| 10 | 190 | 1.06 | 0.97 | 0.95 | 0.68 | 0.63 |
| 15 | 195 | 1.08 | 1.00 | 0.98 | 0.70 | 0.65 |
| 20 | 200 | 1.11 | 1.03 | 1.00 | 0.71 | 0.67 |
| 50 | 230 | 1.28 | 1.18 | 1.15 | 0.82 | 0.77 |
| 90 | 270 | 1.50 | 1.38 | 1.35 | 0.96 | 0.90 |
| 100 | 280 | 1.56 | 1.44 | 1.40 | 1.00 | 0.93 |
| 110 | 290 | 1.61 | 1.49 | 1.45 | 1.04 | 0.97 |
| 120 | 300 | 1.67 | 1.54 | 1.50 | 1.07 | 1.00 |
| 130 | 310 | 1.72 | 1.59 | 1.55 | 1.11 | 1.03 |
| 140 | 320 | 1.78 | 1.64 | 1.60 | 1.14 | 1.07 |
| 160 | 340 | 1.89 | 1.74 | 1.70 | 1.21 | 1.13 |
| 180 | 360 | 2.00 | 1.85 | 1.80 | 1.29 | 1.20 |
| 200 | 380 | 2.11 | 1.95 | 1.90 | 1.36 | 1.27 |
| 210 | 390 | 2.17 | 2.00 | 1.95 | 1.39 | 1.30 |
| 250 | 430 | 2.39 | 2.21 | 2.15 | 1.54 | 1.43 |
| 275 | 455 | 2.53 | 2.33 | 2.28 | 1.63 | 1.52 |
| 300 | 480 | 2.67 | 2.46 | 2.40 | 1.71 | 1.60 |
| 350 | 530 | 2.94 | 2.72 | 2.65 | 1.89 | 1.77 |
| 405 | 585 | 3.25 | 3.00 | 2.93 | 2.09 | 1.95 |
| 450 | 630 | 3.50 | 3.23 | 3.15 | 2.25 | 2.10 |
| 500 | 680 | 3.78 | 3.49 | 3.40 | 2.43 | 2.27 |
| 550 | 730 | 4.06 | 3.74 | 3.65 | 2.61 | 2.43 |
| 580 | 760 | 4.22 | 3.90 | 3.80 | 2.71 | 2.53 |
| 590 | 770 | 4.28 | 3.95 | 3.85 | 2.75 | 2.57 |
| 600 | 780 | 4.33 | 4.00 | 3.90 | 2.79 | 2.60 |
| 610 | 790 | 4.39 | 4.05 | 3.95 | 2.82 | 2.63 |
| 620 | 800 | 4.44 | 4.10 | 4.00 | 2.86 | 2.67 |
| 630 | 810 | 4.50 | 4.15 | 4.05 | 2.89 | 2.70 |
| 680 | 860 | 4.78 | 4.41 | 4.30 | 3.07 | 2.87 |
| 720 | 900 | 5.00 | 4.62 | 4.50 | 3.21 | 3.00 |
| 770 | 950 | 5.28 | 4.87 | 4.75 | 3.39 | 3.17 |
| 795 | 975 | 5.42 | 5.00 | 4.88 | 3.48 | 3.25 |
| 800 | 980 | 5.44 | 5.03 | 4.90 | 3.50 | 3.27 |
| 820 | 1,000 | 5.56 | 5.13 | 5.00 | 3.57 | 3.33 |
I want to create my own table or do my own comparisons, how to?
Assuming the entire interval is linear (mind you, I still only have data for a range of ish 300 HeroPower starting at 232 HeroPower), the damage can be calculated as this:
scalingFactor = HeroPower + 180
dmgPerHit = weaponBase * scalingFactor = weaponBase * (HeroPower + 180)
You get your weaponBase by this formula:
weaponBase = dmgAt(X) / (X + 180)
example Kerry longbow heavy:
dmgAt(305) / (305 + 180) => (dmgAt(305) = 2425) => 2425 / 485 = 5
And if you want to compare how your damage would scale with an increase:
scaling = (oldPower + addedPower + 180) / (oldPower + 180)
But what about armor? It is the same, but a different weaponBase, or rather a modified weaponBase. The same with large enemies or berserk enemies.
And the dummies? Since this game seem to favour integers in many instances, I think the damage displayed on the dummy is the correct one, and the one in the scoreboard is divided by a hundred.
Conspiracy theory
There was something really interesting meantioned in a FS post (Vermintide 2 - Overhaul of damage and stacking damage buff/multipliers):
And as far as I (we?) know, 600 HeroPower is the effective cap, which is reached via level 30 (300 HeroPower contribution) and maximum gear (300 HeroPower contribution), so 600 HeroPower in total. So, why did @Fatshark_Hedge say that 600 HeroPower is the cap for Champion but no cap on Legendary? Shouldn’t “no cap” == “600 cap”? The obivious answer is of course that 600 HeroPower is the “current” cap, so we should expect a higher cap later 
Judging from my table above (which assumes linear scaling outside of the intervals I’ve measured), the possible “real maximum / cap” is presumably either 720 HeroPower (5x dmg of HeroPower 0 and 3x dmg of HeroPower 120), 795 HeroPower (5x dmg of HeroPower 15) or 820 HeroPower (5x dmg of HeroPower 20, with a nice and round RawPower at 1000). In table form:
| HeroPower | Breakpoint |
|---|---|
| 720 | 5x @0, 3x @120 |
| 795 | 5x @15 ← I believe in this one |
| 820 | 5x @20 and RawPower == 1000 |
So how are we getting there? DLC? 
Have anyone of you found gear with iPower above 300?