CDM Gear already has their MTRS light and sling mount in the works for the new Mossberg 590 Shockwave 20 Gauge. Like the 12 Gauge version, the new MTRS will feature 2 sling mounting loops and a light rail and is completely ambidextrous. It can be mounted with the light rail on either side of the shotgun. The bottom sling loop also serves as a hand stop.
The new MTRS for the 20 Gauge Mossberg 590 Shockwave will be released soon. Email 20gaugeshockwavemount@cdmg