A quick, rough measurement...
... shows that my Remington has a slight taper in the chamber, of about 1/2mm. Thinking of practical production, this was probably easier to produce by sinking in a taper reamer or drill than a parallel hole would have been. In fact, I have some 1:48 taper drills that would produce such a hole much faster than drilling + finish reaming.
... and it takes a whopping 36 grains of Swiss No. 2 to fill the chamber up to the level of a seated 454 round ball.
So I reckon the Pietta is OK. But for the sake of accuracy, reduced powder consumption, and your wrist joint, I would avoid a "full load" and proceed as outlined in my previous post.