@Paige HawthorneNot a Jack Reacher story -- it was certainly online, and I've not read any Lee Child. However, your suggestion did set in motion a train of thought; Vegas had me thinking of Protect and Serve by Paul Phenomenon, not the correct story but a similar-ish MC, which gave me a a possible title of Next Door Neighbour for no discernible reason, with a possible/partial authorname of Dot< something > - neither being useful search terms. Scanning the oldest set of bookmarks easily available to me had an ASSTR bookmark for 'Dorsai on the Web' leap out at me (DotW) as being NDN's author (~Dorsai on the root ASSTR address; I posted the site link here for Ernest Bywater back in 2017). I was very surprised when the link worked, and I read through NDN expecting but failing to find the scene. However, loading the next story in that series, Amy & friends, and using ^F search on the page for 'shotgun' led me straight to it:
...I'd paid the quite reasonable check and left a hefty tip for our waitress, and we were on our way back to Abby's car when something tripped a circuit in my brain. Abby and Erika were on either side of me; each got a hard shove to the side, even as I was taking a step forward and crouching down. A fraction of a second later, I heard the sound of a shotgun blast, and felt the air change as the pellets passed over me. Turning as I drew my .45, I saw the muzzle dropping as the guy holding it tried to target me again. He was about halfway there when the slug from my pistol caught him under the chin, lifting him to his toes before he fell backwards.
...She didn't have any trouble getting through the police line, and immediately came over to me to ask what had happened. I told her, and when I was done, she asked "But what happened? What told you he was going to shoot?"
Erika was listening with considerable interest, as was Abby, when I told her "Thinking back on it, I figure it must have been the safety on the shotgun."
Slightly exasperated, Amy said "What?!"
"Only thing I can figure is that I heard it as he flicked the shotgun's safety off, and I just reacted. I didn't even know why, until I heard the blast when he fired."
Erika was looking at me in amazement when she asked "You did all of that just because you heard him flip the SAFETY off?"
I shrugged, and said "Yeah. Like I said, I didn't even know exactly why I was doing it at the time; I was just reacting, on instinct."
"But the SAFETY?"
I shrugged again, and told her "It's a pretty unique noise. I'm packing a .45 and wearing a vest, so I'm kinda on alert, you know?"
So, thanks Paige for the hint that helped, even though it was wrong.
ETA: NDN and others of DotW's stories have underage sex scenes, but if you skip over/edit out these, it's a fair Action/Adventure