You don't take NO as an answer do you?
1) you can't change the FritzBot server c-code so you have to use the hooks for an existing weapon in any of the files that you replace. Some of the files are text script, others specific game format (sound, graphics, compiled...).
2) you have to either make all the files from scratch if you like mapping ...and modeling, ...and sound editing ...and scripting...
-OR-
you have to find an ET mod with the required shotgun files and then copy them over and correct any hooks, filenames and paths to try to get them to work (removing any conflicting files of the original weapon).
Now to give you an idea of how many files;
Code:
in etmain\pak0.pk3
\sounds\weapons\... 6 wav files
\sprites?
\weapons\ 1 file .weap
\models\multiplayer\... varies say 11-28 files .md3 .mdc .tga .jpg .cfg
\models\weapons2\... varies 8-15 files of types .mdc .skin .tga .jpg .cfg
\icons\ .tga
\animations\human\base\ 2 files .aninc .mdx
So much more than 30 files probably (and there could be ones I missed)! And how many have to be modified without errors?
And remember they won't feel like shotguns with no changes to the server code. Suppose you replaced a luger pistol, well then your shotgun would have 8 shells per reload that would kill over a fair distance but have a narrow spread. The damage would be the same as a pistol, and then there would a shotgun silencer??
But if you have ludicrous free time, incredible patience, and are committed to learn by trial and error ...