I have a factory assembled DPMS 20" A2 Classic with a 1x9 HBAR. I believe your DPMS barrel should be 5.56 (with which you can shoot both .223 and 5.56 ammo), but it would be wise for you to verify your chamber with DPMS (mine is marked 223 but DPMS advised me it is really 5.56 .. so much for believing what you read).
That being said, I have shot all of the following types of ammo without issues:
- UMC 223 55gr FMJ
- Fed Am Eagle 223 55gr FMJ
- PMC 223 55gr FMJ
- Federal XM-193 (5.56 55gr FMJ)
- Olympic 62gr (5.56 SS109)
- Win 223 55gr FMJ
- Win 223 62gr FMJ
- Sellier & Bellot 223 55gr FMJ
The FAE is the one most readily available for me, what I have shot the most of, and I find it consistent in performance.
But my personal favorites for 100 yds are XM-193 for 5.56 (a mil-spec round) and S&B for .223 (hottest of all the loads listed at 3300 fps and relatively clean as well).
I have begun using Win 62gr at 200 yds and am impressed so far. 62gr is about the midpoint of the range of bullet weights a 1x9 can handle (50 - 70) and I am testing for accuracy at 200 yds vs 55gr ammo, based on the info in the Ammo FAQ.
I have not, nor do I intend to, shoot anything but brass-cased ammo. My 'smith advised me it is best to use the type of ammo most often used in the country of the gun's origin, and I have no reason to doubt that wisdom.