"Barrels are never bored perfectly straight so even if you set up between centers (which is the most accurate method) the crown will not be absolutely square to the bore axis but is as close as you can get it in a lathe and you will never be able to detect any difference."
I don't understand how you would cut a crown with centers inserted in both ends of the barrel.
I use a tailstock spider to make the bore run true. I use a close fitting long arbor to get a couple of readings over a length to be sure the bore is centered and true.
A piloted reamer would also do just fine. It would tend to self enter.
It does not need to be dead nuts perfect.
