Could be rpm's too low or too high, could be feed rate related, could be that you need a different cutting oil  (I've had a couple of cases where packing the reamer flutes with heavy grease stopped the chatter).  Could be a rigidity problem with the barrel in the lathe.  You might hit a hard or soft spot with the reamer that sets up the problem.   

Chatter is usually from a low frequency vibration in the system, so changing one or another aspect of the job will help to get rid of it.  All of my chamber reamers are six flute, and most barrels I've chambered over the years have been six groove, I've only had the problem a few times, and usually was able to solve the problem by making a change in technique.   

David
Montezuma, IA