It looks as though it has a "sleeve" on the bottom. Only printing is SB 40SW.
I am about half afraid to shoot this till I know what it is... got in a trade...

40 G&A or 40 G&A Magnum?

SB may be Sellier and Bellot ???

Hard to tell but that might be a round made with an SST NAS3 case (Shell Shock Technologies). Check their website and see if you think it matches in appearance. They make some sort of special two-part case that’s presumably stronger, better, etc.

40 G&A Magnum cartridge:

https://revivaler.com/40-ga-pistol-cartridge/

Although hopefully the belt would prevent the cartridge from fully chambering properly in a 40 S&W barrel and therefore the disconnector of most pistols would prevent it from firing, I certainly would not try. I suspect it might be a desirable item for a cartridge collector.

It SHOULDN'T work in your gun, and you don't want it to.

There might be some collectors value to that stuff, you might want to look into it, throw it up on a G-broker auction and see what happens. It might not happen fast, but you never know.

Thanks to everyone... this is just the strangest thing I have come across.... and I'm an old guy, LOL...