As mentioned elsewhere the current DPMS LR-308 receiver picatinny rail height is not compatible with most aftermarket handguards. Most attempt tso replace the factory forearm will result in rail heights that are not on the same plane.
Fortunately there are some new product’s on the way to change this situation.
I have created a new page devoted to help you determine which reciever you have as well as keep you up to date with the current aftermarket options. That page is here.