Is this a factory complete DPMS rifle? From the grouping and erratic nature it appears the barrel may dancing around. FWIW I have pulled barrels off of DPMS rifles and the barrel nut was finger tight!
Check the simple things before you get too crazy. Holding the rifle very firmly(a helper is a nice thing) grab the FSB and try to rotate/twist it. If you detect any movement check the bbl nut. It should be torqued to 40 ft lbs and if needed tighten additionally to line up for the gas tube. Do not exceed 80 ft lbs.
I would re-install the barrel, checking fit of barrel to receiver and the square of the mating surface on the front of the receiver to barrel.
I would also pull the FH and check condition of the muzzle/crown. If it is the least but questionable I would re-crown it.
If you want I am about a 25 minute drive and for me to check it out and correct the issues I mentioned it would cost you less than shipping it to DPMS.