RE the use of a high-energy load as a teaching tool...
I recently graduated college. In the course of my schooling I took well over 2 dozen shooters to the range, many for the first time. Of those I created a couple devotees to the shooting sports who, with any luck, will be lifers. At one point, though, I nearly lost one, and here's how: A girl from Washington state, Rachel, had become a regular shooting buddy over the course of a few months. I started her out on my AR for the low recoil and "plinkability" (didn't have a .22 at the time). Soon after we moved to pistols, and she fell in love with 9mm handguns (liked SIGs in particular, which I'll never understand
). Most of our range trips after that were for handguns.
Eventually she'd become proficient in safe handling and was developing impressive accuracy with a pistol. One time while preparing for a range trip I noticed that I had a small tupperware container filled with old 12 gauge shells that had come from my pockets after various hunting trips. I grabbed the container and my 870 HD shotgun and brought it with. After giving her the basic rundown and observing her firing the gun a couple times, I left her to her own. She was having fun with birdshot at plinking distances while I was shooting in the next lane. Suddenly I heard a boom that exceeded the other shotgun blasts and looked over. She was standing there, stunned, with a look on her face like she was about to cry. The pump on the gun had been flung back from the recoil of the 2 3/4" magnum slug that I'd left in the container and she'd loaded and fired. Turned out there were 3 or 4 left in the container that I'd had leftover after topping the gun off for HD duty and had tossed into the container. I had forgotten they were there, and she didn't know enough about shotguns to realize what they were.
She took a good whack to the shoulder, and has been shy of shotguns ever since. Lucky for me and the rest of the firearms community that it didn't turn her off to guns completely. It dampened her enthusiasm some, and I still feel like crap thinking back on it. Moral of the story: That ain't a valid teaching tool.
RE storage of a dedicated HD gun: I keep mine concealed in a bedroom when I'm home, locked up when I'm out. No kids in my household, and it's in a spot where company ought not be stumbling on it.