You can always try a Thread Alignment Tool (TAT ) it fits in the bore with a land touching shank and is threaded for the die you need ,so the die cuts in alignment to the bore not the outside of the barrel .
One source i know of http://www.cncwarrior.com
They even offer rotary broaches for getting the right Dia. for the threading the muzzle . All without a lathe .
They work well just a pointer in the use of one make sure there is a gap between the TAT`s threaded section and muzzle as you run the die down .
There is more than one way to skin anything !!