The Magpul forend were designed for the 590/590A1 shotties which have a longer action slide tube. You need to upgrade to the 7 3/4" Action Slide Tube to use the Magpul forend.
w ww.brownells.com/shotgun-parts/receiver-action-parts/action-parts/action-hardware/mossberg-500-590-action-slide-tube-assembly-prod43816.aspx
w ww.brownells.com/shotgun-parts/receiver-action-parts/action-parts/action-hardware/action-slide-tube-assembly-7-3-4--prod9246.aspx