The barrel is off to the right by over 1/4".  The upper is a Bushmaster A2, the barrel is an older Old Sarge 16".  The barrel does not appear to be bent, just joined at an angle.  Any theories or methods to determine the cause and correction?  This carbine has broken two bolts thru the cam pin hole, and wears the locking lugs on the bolt like crazy.  It takes about 600 to 1000 rounds for the bolt to go.  Ops

Time to pull the barrel and check the barrel/extension/receiver socket out. Somewhere in the mess, something is out of alignment.

If you can get your hands on a lathe, just chuck the barrel up close to the extension, use a spider on the backside and check out the extension.  If you find that the extension was installed canted (cross threaded or threads not true with the bore) then throw away the barrel and buy a new one (saving the barrel is more work/money than just buying a new one).  

If the barrel checks out, then check the upper barrel socket lip.  If the problem is there, then just face the extension socket lip with an end mill and the problem is solved (just take the high side off).