Yeah a .410 rifled barrel is a 45LC. And that is my point, technically, based on the information here, wouldnt that now be a rifle and subject to the 16" minimum barrel length? But its not, because the firearm was manufactured as a shotgun, it still retains the 18" minimum. (as stated previously, this example cannot be considered a DD, as the bore is less than .50). Same could be said for a 20ga. Rifled 20ga barrel cannot be 16" although it is a rifled barrel.
simple enough! you just mark the barrel "45 long colt" like T/C used too.
and so what if you over cut the throat/forcing cone to where a 410 just happens to fit.
the barrel is rifled and marked as a rifle (the only problem might be when they ask you to shoot it with 45lc rounds.)