Biggest problem in getting the crown square on a counterbored MN is that the outside of the barrel is rarely close to the axis of the inside. Most of them so much so that you have no difficult seeing the difference. Not that it cant be done, but setup would be an EXTREME pita. If it hasnt been too deeply counterbored, you could cut the muzzle back and recrown or have it recrowned by a smith.
No such thing as a stupid question. What is stupid is not asking it.