Ref: uscra112's comments:
Having been a machinist, toolmaker and barrel maker, I can't agree that the external lapping of a lead screw and the internal lapping of a rifle barrel are at all comparable - I've done both; admittedly not so many lead screws as rifle barrels (by a factor of hundreds). Lead screw threads are mechanically generated, and vary little in pitch, while button-rifled barrels, particularly, depend on the pitch geometry of the button itself to generate the rifling pitch down the bore (that is, the button rotates itself to generate the pitch), and can show quite large incident variations in pitch from point-to-point in the bore. This is because the forces on the button are truly enormous, and softer or harder spots in the barrel material, variations in the lubrication of the bore or dimensional variations in its diameter cause the button to skip or stutter in its passage through the bore. It is true that some button-rifled barrel makers attempt to minimize the variations by 'leading' the button - driving it on a fixed pitch mechanically produced, but the forces on the button itself render this, at best, a moderating influence - the torque forces on the button would tie the driving rod in knots, were it not supported by the bore itself - and this can be demonstrated easily enough by taking a 2 or 3 foot length of, say, 1/4" drill rod, clamping one end firmly in a vise, and clamping a visegrip on the other end - mere hand rotation can torque the rod several degrees in either direction.
Barrels made by the other standard processes (cut rifling, broaching and hammer forging) do not suffer from the pitch variations encountered in buttoned barrels. This is not a criticism of buttoned barrels as a class (some of the best barrels made are buttoned), but a commentary on the nature of the process itself.
mhb - Mike