Master of Arms sharpens the Fighter’s two core progression features to their fullest expression.
Weapon Mastery: Choose one Weapon Training group the Fighter has already chosen. The Weapon Training bonus for that group increases by an additional +2, stacking with the bonus already gained from Weapon Training.
Armor Perfection: The Armor AC bonus gained from Armor Mastery at 18th level increases by an additional +1 (a total of +2 above the armor’s own Armor AC value). The Armor Check Penalty reduction and maximum Dexterity bonus increase from Armor Mastery apply even while the Fighter wears armor they are not proficient with; the Fighter gains no other Armor Mastery benefit from non-proficient armor.