Over and over I've seen folks here eithersuggest its possibilityor ask about putting a 7.62mm (or .308/.260/.243) AR upper receiver on a5.56mm (.223) AR lower receiver.

Folks, let's think about this....

Will a .308 (.243, .260, etc.) fit into a .223 magazine?

Since the magazine well in a .223 AR lower receiver is EXPRESSLY designed for .223-length magazines, does it matter that the upper might be designed for .308-length cartridges? Try jamming an M14 or L1A1 magazine into the magazine well of your .223 AR-15 and you'll understand.

See where I'm going here? Neither the suggestion or the question pass the common-sense test.