The third benefit of the Blade Mastery feat (from Unearthed Arcana: Feats) says:
You master the shortsword, longsword, scimitar, rapier, and greatsword. You gain the following benefits when using any of them:
[…]
- When you make an opportunity attack with the weapon, you have advantage on the attack roll.
The third benefit of the War Caster feat (PHB, p. 170) says:
- When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.
If you use your reaction to cast booming blade, does that count as making an opportunity attack as outlined by the Blade Mastery feat, since you are making a weapon attack as part of booming blade?
Best Answer
You would not benefit from the third bullet point of Blade Mastery
Casting Booming Blade, or any other Spell, in place of your Opportunity Attack means you're not performing Opportunity Attack; you're simply casting a Cantrip as a Reaction.
It's worth noting how the War Caster feat words its feature:
Even though Booming Blade involves the use of a melee weapon attack, it does not count as an Opportunity Attack when used this way, and features which normally benefit Opportunity Attacks would not apply to it.