Here's a couple things to look at with the rifle itself. Take the bolt out of the carrier and slide the stripped carrier into the rifle. Check to see in the last 1/2" of travel or so if the gas tube hits excessively on the carrier key. If it does, you can tweak the tube left, right, up, down to ensure the carrier slides in smoothly without moving the gas tube excessively.
Another thing is who assembled the rifle? Ask them if they lapped the front of the receiver before installing the barrel and if they used a locking compount on the barrel extension. Normally I will use the same extension being installed on the barrel and lap that into the receiver for good contact, then after the barrel work is done I use red Loc Tite on the extension when sliding it into the receiver and a moly grease on the barrel nut threads. In talking with some equipment specialists at Rock Island, some of that information came to light in an issue Knight's Armament had with their M110's and what they did to fix it. One of those things was to epoxy the barrel extension into the receiver. Receiver fit is generally considered to be not so critical, but an accu-wedge will solve any questions with that, or you can bed the upper to the lower to remove any play.
Just some thoughts.
