This is not how the armour, nor any other item increasing Critical Damage works.
Let's consider a 50 - 100 DMG gun, maxxed.
Damage will be rolled between 50 and 100, of course. The average of all your hits will be 75, which is (MaxDMG+MinDMG)/2.
If the game rolls a critical hit, you add the Maximum Damage of your gun to the "normal damage" you rolled previously. In average, you will roll 75 average damage in this gun, and add on top 100 damage every time you critically hit. This mean a critical hit will deal an average of 175 damage on this gun. 175 isn't double the damage dealt as you're claiming, but 2.33 times it.
Now, let's add 60% critical damage. This modifier is applied only on the Critical Portion of the attack - which is, Max Damage. Here, 100. The Normal Portion, of 75, is unaffected. Therefore, a critical hit under the effect of the armour will deal on average 75 as a base roll yet again, and add on top 160. This gives an average of 160+75 = 235 damage every critical hit. 235/75 = 3.13 times the average damage you deal with the gun.
Therefore, the real average increase in critical damage you get is 235/175 = 1.343 times the average normal crit.
That's a 34,3% increase.
- Inistae