Fro those who have to have the best, foolproof method, the Brownells jig will do it- if care and some modicum of skill is used.
A small square and mill file will also do it as also noted in the answers above.
ONCE the muzzle is square, many times I have use a tapered stone with emery or paper around it to cut a 60 degree angle(or whatever they are), then the old thumb and emery or paper method- which ALWAYS gives a perfectly centred radiused and smooth crown. You simply turn the barrel every 10 seconds or so- it always comes out perfect.
Doing all of the work needed using a lathe is so simple and fast- maybe 3 minutes tops for the whole job.