Note this bonus does not scale with the weapon's base damage, unlike the Mirrors speed bonus. It depends only on your Iron score, meaning faster weapons (Deck, Forward) benefit more than slower ones (Aft). Also note that at the maximum possible level of ~250 (200+bonuses) it's equal to about 40, making the damage differences between strongest and weakest weapons barely noticable.
Iron can change temporarily or permanently. Ship and weaponry upgrades can quickly increase total Iron, but not base Iron.
You can increase or decrease Iron temporarily by having the following: