I have a 20" Faxon match barrel on my Aero Precision M5 that's been pretty disappointing...about 2.5" at 100 yards, regardless of ammo used, including Federal 168gr GMM.
After reading this thread, I just ordered a 18" Criterion barrel from Fulton Armory, which will hopefully fix the problem.
I hope it works...I'm getting pretty tired of wasting my time.