There is a *slight* mis-alignment where the barrel is attached to the upper receiver, usually caused by the barrel rotating slightly as the barrel nut is tightened down. This is exactly the source of the problem with all of the Bushmaster "overtorqued" barrels.
You fix the problem by loosening the barrel nut and retightening it while preventing any barrel rotation. There is a pin on the barrel that fits a slot on the receiver. It may be necessary to file a bit off the pin, or if it needs to go the other way, to use a shim between the pin and the slot. A little square cut from an aluminum can works great.
Of course, you need a vice, action blocks, punches, and a barrel wrench at minimum. A gas tube wrench is also nice.
-Troy