Many years ago I read about this trick in a Brownell's book.
When you own a pump action shotgun you sometimes find yourself wondering with just a tiny bit of paranoia if you might have foolishly left it with a shell in the chamber the last time you put it away. The only way to check (with most older guns) is to carefully ease back the pump just a bit and peek, which isn't really a situation I want to put myself into.
Also, there have been accidents where someone not knowledgeable about gun safety will pick one up or have it handed to them and the first thing they do (it's like some sort of weird instinct) is operate the slide - often Terminator style - thus loading the gun and creating a hazardous situation.
This simple little piece of PVC pipe can help prevent such foolishness.
The device is nothing more than a 4" piece of 1" PVC pipe with about a 3/4" wide slit lengthwise. Round off the corners and edges, drill a couple of holes and loop a strong cord through it. Done.
It pops on and off the magazine tube. I keep it located between the slide handle and the loading port, but you could also use it to lock the slide open rather than closed, though it would keep the firing pin spring compressed. Either way, as long as it's in place the slide cannot be operated.
Stated very simply - my rule is that whenever this little DIY item is on the gun, the gun's unloaded, and the only way to load it is to make a deliberate but fairly easy action to remove this piece before operating the slide.
I'd be happy to answer any questions, but I will note that if you can't look at it and intuitively understand what it does, you REALLY shouldn't own a gun anyway... :-)