If you have the spell sniper feat (doubles range of spells), and you use a weapon with reach, can you cast booming blade and hit a creature at 10ft?
Can you also combine this with the feats Polearm Master and War Caster to make a booming blade opportunity attack when a creature enters your reach (of 10ft)?
As of Tasha's Cauldron of Everything, booming blade now has a range of Self. This means Spell Sniper won't have any effect on it (you can't double a range of Self) and War Caster won't let you use it in place of an opportunity attack (War Caster requires the spell to target the creature).
Note that this answer is for the versions of Booming Blade before what was published in Tasha's.
I will leave the body of this answer as is, as not everyone owns Tasha's.
Note the range on Booming Blade is 5 feet, so following Spell Sniper it should be doubled to 10 feet.
Jeremy Crawford has tweeted on this topic, about a similar spell Green Flame Blade.
Q: What if he had the Spell Sniper feat? The spell requires an attack roll, would its range thus be 10ft, allowing the whip?
A: Spell Sniper does work green-flame blade.
Booming Blade requires the Cast a Spell action, and only has an attack as one of the components. Opportunity attacks must be attacks, not spells.
To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature.
From the feat:
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.
Since Booming Blade is a spell with a cast time of one action, and only targets one creature, you could cast the spell as your Opportunity Attack with the Warcaster feat.
While you are wielding a glaive, halberd, pike, or quarterstaff, other creatures provoke an opportunity attack from you when they enter your reach.
Combining Warcaster, Polearm Master, and Spell Sniper would allow you to use Booming Blade at 10 foot range as a creature enters your reach.
Note that even though Booming Blade's range is increased to 10 feet, since it requires a melee attack, your weapon must still reach that far to use it at this range.
The booming blade spell has a range of Self (5-foot radius), and I can see no reason why this can't be doubled to Self (10-foot radius) like any other spell.
However, even if we can double the range, the doubling is ultimately useless as the spell explicitly states:
[...] make a melee attack with it against one creature within 5 feet of you. [...]
Thus, increasing the range won't actually help us to target creatures that are farther away. This is similar to the case with the counterspell spell.
A spell cast using War Caster must target only the provoking creature so if booming blade targets the caster, it can't be used with War Caster.
To borrow from an answer to a similar question and the intention for these sort of spells (a tweet from Lead Game Designer Jeremy Crawford):
- "Self": means that the spell targets the caster
- "Self (XYZ)": means that the spell is an area-of-effect spell, whose point of origin must be the caster
A note about D&D spells with a range of "Self (XYZ)": the parenthetical—which says "5-foot radius," "15-foot cone," or something else—means you are the spell's point of origin, but you aren't necessarily its target. You're creating an effect that originates in your space.
So such a spell, at least according to this answer and Crawford, does not target the caster and thus can be used with War Caster. Crawford has even explicitly explained the interactions between War Caster and booming blade:
Booming blade works with War Caster.
So the intent here is clear, and looking into the actual rules we do find the following:
Spells that create cones or lines of effect that originate from you also have a range of self, indicating that the origin point of the spell's effect must be you.
So Self (XYZ) indicates the point of origin and not the target of such spells and we're done, yay.
Unfortunately, this rule doesn't explicitly address spheres and a different rule states that "a sphere's point of origin is included in the sphere's area of effect". Because of this, it is unclear to me how this combination should work if we stick to reading the actual rules. There are countless questions about spell targets and AOEs across this site with people arguing every which way with plenty of upvotes held by everyone. This is an incredibly messy part of the rules.
At the end of the day, a GM is free to rule however they want. At my own tables, booming blade does not target the caster, its range can be extended by Spell Sniper, and it can be used with War Caster.
