That's also not correct, Tuco ^.^
For example...
If that was the rule, then you could never counterspell someone attempting to counterspell your fireball, the exact example given in the handbook... since that counterspell would be a second levelled spell that you would be casting on your turn.
The rule itself is very arbitrary, and very stupid, and it legitimately adds nothing to the game for existing, and absolutely SHOULD be ignored and dispensed with... because all it ACTUALLY does is trip player sup and limit them unnecessarily. It doesn't prevent any major exploits for existing, and it doesn't add to the fun and enjoyment of the game.
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Imagine, if you will, two scenarios:
A 17Wizard/2Fighter Casts Meteor Swarm. Across the field, an enemy mage was waiting to counterspell the first thing that powerful caster did... The mage casting metoer swarm sees them, and, without interrupting their calling of meteors, expertly counterspells their attempted counterspell. Meteors rain down and decimate the field, and then, because they were somewhat irked at this, the powerful wizard then action surges, and levies an eighth level disintegrate at the impudent little upstart that had tried to counterspell her - the poor enemy caster can't counterspell this as they've already used their reaction, and they are disintegrated with great prejudice.
However... imagine scenario two:
The same powerful, high level wizard/fighter steps forward, but decides to raise a quick shield of faith over themselves before they begin calling their meteors... Across the field, and enemy mage, waiting to counterspell the first thing the powerful caster does... counterspells their shield of faith. The powerful mage, who always follows the rules as written in the handbook.... CANNOT counterspell the enemy mage: Counterspell is a levelled spell, and it is their turn, and they have cast a bonus action spell, so they are NOT ALLOWED. Moreover, they now CANNOT cast their meteor swarm. EVEN IF they Action Surge now, they STILL CANNOT cast their meteor swarm, or their disintegrate, or even a measly little magic missile at the enemy caster who foiled them. They throw a firebolt in disgust... it's all they can do.
This is the same mage, with the same abilities, following the by-the-book rules.
The bonus action spell rule is not a sensible rule, and it doesn't serve a legitimate value. There is no break in balance to allowing spells to be cast according to their required turn economy and nothing more; there actually aren't that many bonus action spells, and none of them break the balance of the game to accompany a normal levelled spell any more than many, many other things that other classes can do with impunity. (like, for example the fighter/warlock/pally who can nova off 11 spell slots in a single turn perfectly legally.)
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Sorcerer, and Quicken Spell, IS the one exception to this, and it should be an exception. Quicken is the problem for which the above rule really exists, and it should apply to Quicken exclusively, not hobble all spellcasters everywhere.
Quicken should be written:
"When you cast a spell that has a casting time of 1 action, you can spend 2 sorcery points to change the casting time to 1 bonus action for this casting. If you cast a spell as a bonus action using this Metamagic, you can't use an action to cast another spell during the same turn, unless it is a cantrip."
Implement quicken that way, and remove the arbitrary bonus action spell overarching rule, and you'll have a much better game.
