RE: NEED HELP, QUICK!!!
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.
I have yet to see a .308-length upper receiver that's designed for a .223-length lower reciever. The AR-15/M-16/M-4 has been adapted to cartridges such as the 7.62x39, the 6.8mm SPC, and the 9mm (all shorter or similar in length to the 5.56x45mm NATO), but a .308-length cartridge requires a .308-length (AR-10) lower reciever.