Just got my M&P ar 15 from S&W and there is no caliber designation on the receiver. There is nothing under the "model M&P-15" where the manual says it goes. Only on the barrel does it say 5.56. Ive noticed others in this forum are the same way. No biggie. Just curious. Cant wait to heat it up!

probaly because you can change uppers to a different caliber

This has been my understanding as to why many manufacturers don't list caliber on the lowers.