When I had a Smith & Wesson M&P 15, I cased it with a Bulldog Assault Rifle Case. It is a well made soft case that floats and has great padding to protect your firearm.
The Bulldog case is made from thick heavy duty nylon that is water resistant. The zipper is fill length and heavy duty also. You can feel that it is made to last and stand up to abuse. The inside is a padded brushed tricot lining. It comes with an adjustable shoulder strap and an outside pocket that holds 4 AR-15 magazines. It holds the magazines securely.
I bought one in OD and it looked great. I bought the 40 inch one for my M4 AR-15 and it fit perfectly. I really like the Bulldog case. Very nice product.
I definitely recommend it for anyone needing a M4 rifle soft case.
Best price and selection is at Amazon but you can also get one at Midwayusa.com.