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Booming blade is a Sword Coast Adventurer's Guide cantrip that allows the spellcaster to make a melee weapon attack with their weapon and add additional power to the attack:

You brandish the weapon used in the spell's casting and make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack's normal effects and then becomes sheathed in booming energy until the start of your next turn. If the target willingly moves 5 feet or more before then, the target takes 1d8 thunder damage, and the spell ends

Booming Blade, Sword Coast Adventurer's Guide, pg. 142

The Twinned Spell metamagic permits a Sorcerer to take any spell that targets only a single creature, and cast it twice, targeting a different creature.

When you cast a spell that targets only one creature and doesn't have a range of self, you can spend a number of sorcery points equal to the spell's level to target a second creature in range with the same spell (1 sorcery point if the spell is a cantrip).

To be eligible, a spell must be incapable of targeting more than one creature at the spell's current level. For example, magic missile and scorching ray aren't eligible, but ray of frost and chromatic orb are.

Twinned Spell, Player's Handbook, pg. 102

It has been commonly accepted that this is a valid use of this metamagic/cantrip combo, but a recent Errata to the spell has changed its range to "Self (5-foot radius)". Is this spell still eligible to be Twinned?

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A range of Self with an area of effect is a further specification. Booming blade still has a range of self.

Booming blade has a range of Self (5 foot radius). Twinned spell says:

When you cast a spell that targets only one creature and doesn't have a range of self

Xirema's answer has alleged that Self (5 foot radius) is not a range of Self with respect to Twinned Spell. This is not the case. The spell still has a range of Self, and an area of effect is further specified. This is clear from the rules for spellcasting, which state:

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 (see “Areas of Effect” later in the this chapter).

This rule tells us that spells having a range of self together with a further specification of an area of effect still have a range of self, so are not eligible to be twinned.

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The spell cannot be twinned

I will use the video Sage Advice - Targeting Revisited from a Dragon Talk episode to support the explanation, including links to the relevant timestamps.

In the book Sword Coast Adventurer's Guide had a Range of 5 ft, but it has now officially changed to Self (5 ft radius), as it appears in Tasha's Cauldron of Everything (link to timestamp).

This is intentional and previously the Range was wrong (link to timestamp).

In the video, we have the following question and the answer is no (link to timestamp):

Question: How does this interact with metamagic specifically twinning of spells

Answer: This does make the spell ineligible for being twinned because a twin spell does not allow you to twin a spell with a range of Self. Because these ranges have the word Self in them they are not twinnable and that's by design. Honestly these never were meant to be twinnable because again their range was was actually always incorrect.

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Yes, Booming Blade can be twinned, because a range of Self is not the same thing as a range of Self (5-foot radius)

When a spell has a range of "Self", it definitionally targets the caster themselves, and no one else. For example, a spell like armor of agathys (PHB, 215), which places Temporary Hit Points on the spellcaster (and cannot be placed on anyone else), has a range of "Self". Conversely, a spell like arms of hadar (PHB, 215), which produces an area of effect, very explicitly does not target the spellcaster, but instead all the other creatures within the radius surrounding the spellcaster; it has a range of "Self (10-foot radius). So by example, we can see that these two ranges do not represent the same concept in 5th edition D&D.

Naturally, a spell like arms of hadar cannot be twinned: the requirement for Twinned Spell is that the spell must be "incapable" of targeting more than one creature, and this is obviously not true for arms of hadar. But booming blade targets a single creature: the creature being attacked, and it doesn't target the caster, because "Self (5-foot radius)" isn't the same as "Self".

As a result, booming blade, post-errata, is still a valid spell to use with the Twinned Spell metamagic.

The Spellcasting Rule about "Spells that create cones or lines of effect" does not apply

This is the rule in question:

The target of a spell must be within the spell's range. For a spell like magic missile, the target is a creature. For a spell like fireball, the target is the point in space where the ball of fire erupts.

Most spells have ranges expressed in feet. Some spells can target only a creature (including you) that you touch. Other spells, such as the shield spell, affect only you. These spells have a range of self.

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 (see "Areas of Effect" later in the this chapter).

Once a spell is cast, its effects aren't limited by its range, unless the spell's description says otherwise.

Range, Player's Handbook, pg 202

The reason here is that this rule specifically calls out spells that create cones or lines of effect as also having a range of "Self", but booming blade does neither of these things. It simply makes an effect that targets a single creature within range.

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    \$\begingroup\$ I'm not sure I quite agree with that logic. \$\endgroup\$ Commented Nov 11, 2020 at 21:34
  • \$\begingroup\$ FWIW, Jeremy Crawford sorta agrees: "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. " and later stated: "Booming blade works with War Caster." (which is different, but related) \$\endgroup\$ Commented Nov 11, 2020 at 22:38
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    \$\begingroup\$ I thin kit would help if you addressed the spellcasting rules which state "Spells that create cones or lines of effect that originate from you also have a range of self". I don't see the rules as saying that "self (5')" is different to "self". \$\endgroup\$ Commented Nov 12, 2020 at 7:52
  • \$\begingroup\$ "Thankfully, everyone knows I'm not a persnickety person" -Xirema ;) \$\endgroup\$ Commented Nov 12, 2020 at 13:29
  • \$\begingroup\$ I will fully acknowledge the persnickety-ness of my addressing of that rule. Call it a protest against WotC trying to take my toys away, if you like. I do maintain that the usage of the term in the 2020 errata is in contradiction with how it was used in the original 2014 printing of the PHB, but "I don't think they really meant X when they say X" can't really be justified in a Rules-as-Written answer. \$\endgroup\$
    – Xirema
    Commented Nov 12, 2020 at 13:44
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So if booming blade targets self, that would make it invalid to use with war caster feat, as war caster states the spell cast can only target the creature that provoked the attack of opportunity, and " self" is not said creature. Since Jeremy Crawfod officially stated that Booming Blade works with War Caster, it must not actually target " self ". Either it targets " self or it does not. If it does then is invalid for both features, twinspell and war caster, or it does not and is valid for both. You can't have the same targeting mean two different things. Now as to how they intended it, could be that is how they want it to work, but if so they need to change the wording to fix the issue

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    \$\begingroup\$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. \$\endgroup\$
    – Community Bot
    Commented Oct 26, 2022 at 15:50
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    \$\begingroup\$ Welcome to RPG.SE! Take the tour if you haven't already, and check out the help center for more guidance. This seems more like an attempt to comment on the other answers, rather than an attempt to independently answer the question – in particular, it seems to be commenting on/complaining about the inconsistency between the rules and what Crawford's tweets (which are not official rules) state, rather than clearly answering the question directly. \$\endgroup\$
    – V2Blast
    Commented Oct 26, 2022 at 16:35
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    \$\begingroup\$ I respect the intent of this answer, but we've been making a practice in the last few years of not using Jeremy Crawford to justify answers, and using Crawford's (at this point well-documented) lack of consistency is equally probably not a great way to answer questions. \$\endgroup\$
    – Xirema
    Commented Oct 26, 2022 at 20:07

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