Ok, a couple years back i got shorty upper for my preban dpms. It has a 11.5" barrel with 5.5" permanant flash suppressor.  It was pretty much new, shot probably 10 or 20 rounds. Anyway this is the most inaccurate ar i have ever shot. 4inches at 50yds. from a rest, thats on a good day. I tried 3 different scopes, even used a kns x-hair front post for awhile, many types of ammo, etc. I searched this site and have'nt found much info on a barrel just being inaccurate. I don't remember exactly where the barrel is from, but it was either M&A parts or Model-1. The kids like to shoot this rifle so i would like to keep it. My Question: What uncommon problems can i check for?.....btw i had a trigger job done already. Thank you for your thoughts.

That kind of accuracy.... its gonna be a barrel defect.  Most likely a damaged crown.