Watershed has developed a new waterproof weapons bag specifically for the Mk18 carbine.
As you can see it incorporates a waterproof zipper as well as an oral inflation tube and laser cut PALS webbing along with a haul loop.
Internally to the this seam sealed bag, there is a tie down point for the weapon.
Units and agencies can procure products seen at Warrior West by contacting ADS, Inc.