Strange though it may seem (because of it appearing in the weapon section) that sound entry is not actually used when the shotgun is fired. The shotgun sound has a cocking noise at the end of it so there is no need for a second cocking sound.
weapons/shotgr is actually used for the cocking sound you hear when the intermission screen between levels is finished and the next level is loading.
I didn't realize that when I was making the names to use in SNDINFO, or else I would have given it a different name. Too late now, I suppose. Maybe I'll put a comment in pointing this out.