Be extremely careful in opening a choke with no guiding parallel. Your reamer "MUST" be given some form of mechanical guidance to keep it aligned with the bore. Don't let anyone tell you that you can simply use an expansion reamer from the muzzle & "Eyeball" it.
This has been done quite successfully when there is already a parallel section to follow, but a continous taper to the muzzle is an entirely different proposition.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra