By the timeline you're describing - this puts us into Season 9 of Five-O.

There's a couple of possibilities. You can check out Mr. Mike's Five-O page to find all sorts of stuff! It's here: https://www.fiveohomepage.com

Now, onto Season 9. Like I said, there's a few possibles.

S09E08 - Let Death Do Us Part is about as strange as it gets. A woman was killed but she wasn't shot by her son but there's a lot of weirdness in it and I know at least one person is shot.

S09E13 - Man In A Steel Frame is an excellent episode. The victim is shot but again, not by her son. We see things in flashback mostly in this episode. A pistol was used, not a shotgun.

S09E15 - Elegy In A Rain Forest is probably the closest we're going to get. A shotgun stolen from a guard is the weapon of choice for the villain. One of the first sequences in the episode has the guy escaping a prison van during transport and pulling the trigger into the camera (i.e. the guard he kills). The villain is not a nice person - so your grandmother's comment fits.

The last one I can think of is S09E22 - See How She Runs. A young woman is essentially framed and hides in a cult. There's a lot of shooting in that one too.

Hope this helps in the search.