I built two English Sporting rifles for a fellow in Kansas, and the first has two barrels - .32 and .40 cal. and the second is a .54.
I bought Green Mountain's 42" swamped barrels and cut them at 32". This gave me a straight taper that is perhaps a little radical (more than necessary) but was the easiest and cheapest way to go. As a warning, you'll need a high front sight if you use this method. BTW, there was no run out - the bores are dead centre at the cut-off.