As far as someone flying backwards, through a window, etc., it's crap. Always has been.
What brought this to mind is an episode of 'Mythbusters'. They took a pig carcass, hung it up on a balanced mount, and shot it with various firearms to see if it would be thrown back. Nope. Not with a .308, not with a burst from an MP5, at the end either a 12-guage slug or 00 buckshot transferred enough energy to make it fall off the mount, but that's all; it did NOT go flying backward, not a bit.
Peter Capstick, in various of his books and articles, explored the idea of 'knockdown power', finding it to be crap. Bullets just don't work that way. A bullet penetrates, so the energy is used in pushing it through the target and(in the case of soft tissue) damaging the tissue. If you took the energy tied up in a .357 Mag bullet and put it into a steel plate, say, a foot square, striking a man it would be able to knock him backwards, but that's a whole 'nother thing. Capstick said that in his experience he'd never seen any animal of a size greater than 40lbs actually knocked over/back by any cartridge, including big express-rifle stuff.
I think that if you were talking about a shotgun hitting somebody at close(a few feet) range, that might be able to physically shove the target back, but that might be equally from the press of expanding powder gas as well as the shot. And it would probably have to be shot; a slug would penetrate, shot would be more likely to transfer its full energy to the target.
This is one of the reasons I don't really like L. Sprague deCamp. He's an author. I originally didn't like him because he 'edited' a lot of the Robert E. Howard 'Conan' stories, and his 'editing' often involved rewriting the damn thing. Then, he wrote some short stories collected in a book called 'Rivers of Time', about a guy running safaris back in time. In the first story the character Rivers is laying down the law, saying that if you want to go back and hunt dinosaurs the only rifle you'll be allowed to carry is a .600 Nitro Express, because it's the only gun that can knock a dinosaur down. Which is followed, in at least one of the stories, by description of a dinosaur being knocked off it's feet by the impact of a shot. Besides messing up the stories for anyone who does know anything about guns, it spread bad information(done by a guy who's supposed to be serious about science).
Oh well. Now I can just wait for the next 'investigative report' from some idiot journalist who doesn't bother to find out that 'assault rifles' aren't any more powerful than anything else. Oh well.