There are several ways:
1) get a TAT and die from dyna-systems.
2) remove the front sight base and pass the barrel through your headstock with just a few inches of barrel out past the chuck.
3) make a jig that fills and centers on the chamber, but provides shaft out past the end of the upper receiver, and has a dog to turn the piece.
4) use a fixture for the upper receiver that takes the place of the bolt carrier, and uses a key in the charging handle slot.
1, 2 and 3 are about as precise as you'd get unmounting the barrel and turning with centers and a dog.
I've done 1, 2 and 4 before, and had no problems with concentricity. You need about a 1" through hole in your headstock to do 3, and it also involves removing the front sight base, so I normally wouldn't bother. When turning a barrel between centers, use a small soft bit of copper, brass or lead to protect the crown (and throat, if applicable) between the center and the crown or throat.
Any turning involving leaving the receiver or front sight base on needs to go at a slower speed than normal since the piece isn't balanced along its spin axis.