What twist is that barrel? 1:9 or 1:8? I have a DPMS 20" bull barrel, in a 1:9. I haven't shot reloads through the barrel, just factory loads. It really likes the heavier grns, from 68 to 75grn., usually shoot Blackhills red box through it, when I can find it. But at the prices lately, I may start reloading for it. Accuracy wise, with mine, I broke the barrel in, have it mounted on a bipod, free float forearm, 4-16x50 scope, and a 2-stage Rock River trigger that was re-worked by Bill Springfield. If I do my part, control the breathing, sight in, and squeeze off the round, I can keep under 1 moa @ 100 yards easily. Out to 300 yards, I can still keep it around moa +/- .25. The farthest I have shot that particular setup is out to 500 yds on a range. Even with a 13mph cross wind, on a 73 degree day, sun at my back, my best grouping measured 6.125". Not too bad for an upper I only paid $489 for on sale. I would guess the 16" could come close to that. But with the shorter barrel, you won't get as much velocity out of it, which could open up your groups a bit. Best bet, find a good range of ammo, from varmint rounds up to the heavier rounds and shoot for groups. that will help to give you a base to start building around for your reloads.
Hope that helps a little bit. There are a few on here that I believe have that same setup, so I'm sure they'll chime in soon. Good luck and good shooting.