I just realized Massis was talking about making the stem and shank faces parallel---removing any "light gap"---
not the mortise/tenon fit. (The word "fit" was what tripped me up)
Fixing THAT is an even bigger bitch.
90% of it can be avoided in the first place, though, and won't cost you a dime. No need to make a video, I'll just tell you now:
Let the stummel rest on a shelf for a month or two between every fabrication step.
That's it.
Really.
It's because the moisture in a briar block is never evenly distributed, but follows a surface area to volume dependent curve. And every time the ratio changes---which is every time the block is shaped and/or drilled---the moisture inside the block shifts in response. The moisture of a carver's hands even figures into it. And wet briar is larger than dry briar.
The net result? Drilling the mortise and facing the shank on a briar block that was recently roughed in guarantees there will be a light gap when the moisture in the block stops migrating a couple months later.
End of story.
Removing that gap once it exists on a "shape stable" pipe should only be repair work. A carver who follows good "moisture protocol" with his blocks should never have the problem.
UFOs must be real. There's no other explanation for cats.