So you know in the future Tata, the lower most times will not have a caliber stamped on it. With all the different uppers that you can get in different sized calibers, you would have a book stamped on the side of the lower. That's the beauty of the AR style rifles. You can change what your shooting very rapidly.
Did you buy this firearm new and used. With new ones you don't have to worry about uppers getting swapped around. If it is new, you can call the S&W CS. and give them the SN off the lower and they can tell you how the firearm left the factory. They can tell you how the lower left the factory anyways. That barrel is going to be stamped somewhere on it. If you bought it at a gun shop, take it in with you and ask them pop the hand guard off so you know what you have. That's just good customer service. From the looks of it, My money rides on you having 5.56 with a 1/9 twist(maybe)

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15T,15-22,5946 & some not S&W