17.2mm converts to .677". In the gauge system this would be a 16/1 (.674). If 16ga wads do not have a good tight fit use 14ga ones, the wads want to fit snug to prevent the 2nd bbl wads from moving on firing the first, particularly so with the overshot wad. Remember also it may not still measure that dimension, it may have been honed or polished over the ensuing years.
As Jim said the vast majority of ML'rs were bored cylinder with no choke.

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