So in my non-scientific and statistically invalid review of all the auction sites, in which a distinct minority of the M21 listings have high resolution images of the barrel flats, I've yet to find one that looks like the subject barrel or Dewey's. They look like the examples posted and here's another:

[Linked Image from photos.smugmug.com]

I'm not qualified to discuss the design defects of the M21 but believe inexpert joining of the dovetail joint is found in only few. So the problem could be:
1. sampling error - certainly possible
2. the M21s with breech failures have been trashed or rebarreled
3. cherry picking to make a point