This rifle case fits my M&P 10 perfectly with the stock collapsed.
Five 20-round MapPul mags fit on the side as well. I bought this
same case at Amazon a year ago for $45. It's no longer available
at Amazon, I assume it's discontinued because of the length being
too much for a carbine, even a carbine fits fine if you extend the stock.
It's made by Allen, and very well padded all around.
http://www.fieldandstreamshop.com/p/smith---wesson-m-p-tactical-rifle-case