I was employed as a machinist for 35 years & in the course of those years reamed a lot of holes. I can assure you that an adjustable reamer will not center automatically in a hole "Unless" there is already a parallel section for it to follow.
Try the above advise on a taper choke & be prepared for it to hit anywhere but where you look. You can expect good results on a conical parallel choke, just be certain & discern which type you have prior to taking out any metal.

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