I have already read it. Either tell me what you think bolsters your argument or don't ask again.
Now, look at the two lowers and tell me what is different. Remember that rant you went on about 5.56? Only one of those was chambered for 5.56. There are also more differences. You can shoot the 5.56 thru the SP1 but you could have problems with it.
One was designed for the battlefield and one was not.