Some really nice suggestions so far –––– I'll throw in a low buck favorite ––-i really like Magpul's MOE stuff . Your DPMS was designed to be lightweight and the MOE stuff will not add any weight to it at all. It looks pretty slick and you can install a buttstock , pistol grip and the handguards for around 150 bucks for all .
They have rudimentary slots for attaching things, but if you really need a hardcore rail system –––– stuff from VLTOR, Troy and Larue is the place to go ––-
their stuff is all over this website in plenty of pic threads, but here is mine with an ACS buttstock, and MOE hand guards pistol grip and mags
This pic is actually a carbon copy of your rifle except with Green MOE stuff (CTR stock ) - forgive the FDE MBUS and mag –– LOL. I did eventually replace the stock bolt with a BCM bolt on this rifle though