These can be tricky. A few years ago when I built my first one I did all the research and asked about what works and what doesn't.
Like you, I settled on the DPMS platform with a POF lower and a Fulton Armory upper.
Another Calgunner posted how his nearly identical setup really liked PMAG's, so that's what I bought.
To date, it is not only a tack driver, but utterly reliable. In over 1000 rounds, I've never had a FTF or FTE.
You might ask around to see what your setup prefers, but PMAGS's would be a good place to start.