Master Armorer (Ex)Source Armor Master's Handbook pg. 15
The fighter can use his base attack bonus in place of his ranks in the Craft (armor) skill. The fighter need not be wearing armor or using a shield to use this option. The fighter substitutes his total base attack bonus (including his base attack bonus gained through levels in other classes) for his ranks in this skill, but adds the skill’s usual ability score modifier and any other bonuses or penalties that would modify that skill. Additionally the fighter is treated as having the Craft Magic Arms and Armor and Master Craftsman feats, but only for the purpose of making magic armor. The fighter does not need to meet these feats’ prerequisites.