Shell archives (“shar files”) used to be a popular way to distribute sources, but it isn’t used all that much today. In any case, we have some commands to deal with these:
Unshars the current series (gnus-uu-decode-unshar).
Unshars and saves the current series (gnus-uu-decode-unshar-and-save).
Unshars and views the current series (gnus-uu-decode-unshar-view).
Unshars, views and saves the current series
(gnus-uu-decode-unshar-and-save-view).