As Chuck points out, most "lines" in drafting are ideally a one dimensional (length)representation or convention for a two dimensional plane or section thru an object. Could be that sometimes the "real world" plane represented needs to be leveled but an even more important attribute is planar "truth": a plane which is not twisted or warped ("in winding"). By architectual, boatbuilding and aircraft standards, the watertable of a breaking gun is a very small plane but I would assume it's planar nature does bear on the successful assembly and aligment of barrelset and action.

jack