How do you actually draw file a barrel?
Hold the file with the tang in one hand and the tip in the other, roughly perpendicular to the barrel, and push it away from you along the flat (pulling toward you works too). In practice the file is often angled slightly rather than held perfectly perpendicular, and you feel for a sweet spot where it peels metal off quicker. Holding the file close in rather than spread out reduces the tendency for it to rock.