The overall length of the standard M&P 15-22 is about 34". This case is 29" long, so it would not fit a fully assembled rifle.
Carly is correct and the purpose of the case is to carry a (broken down) AR. Where the barrel and upper are on one side and the lower and stock are on the other. The small size is what makes it discrete. This is a well padded case with a removable full size divider separating the two halves.