Cant is not that bad, but really need to see a photo inside your upper receiver of the barrel lugs to the center line of the gas tube channel and center line of the front take down lug.


Hence with the slight amount of cant that I see being adjusted for at the rear sight to zero, should have enough room to slip the barrel in the upper receiver barrel socket, that will both allow a dead center rear sight zero, and the lugs not so over rotated that its going to cause problems with the bolt lugs entering and exiting the barrel extension.

FSB is not canted on the barrel (compared to the barrel extension); its the entire barrel that is rotated in the upper receiver barrel socket that is causing the problem instead.






The easy fix, lay just the upper receiver in your lap (off the lower receiver), get a leather mallet that will not mar the steel in your strong hand, hold the receiver with your weak hand at the upper receiver, have the FSB part of the barrel just past your leg, and strike the FSB just above the barrel to slip the entire barrel with barrel extension in the upper receiver barrel socket.  If you need a index mark of how much to slip the barrel in the upper, just look at the feed ramps on the barrel until they are aligned.

Hence the barrel with barrel extension are just friction fit in the upper receiver barrel socket via the barrel nut, and you can slip the entire barrel in the socket without having to loosen the barrel nut.