What's the most overrated handgun/rifle cartridge?
It would be easier to list the most underrated. I say that because most rounds do at least something very well or they would have faded away after a few years no matter the amount of commercial hype. This is the case with the 357 Sig. Not a bad round but clearly over hyped for a few years. It has faded away to a bit less than a niche round. It's not overrated if anything it's under rated.
Over rated handgun round. My pick is the 44 Spl./44 magnum combo. I like the 44 Spl. a lot personally. It will do about all that a 45 acp will do from a wheelgun and do it well for a bit longer distance in my experience. It also does it without moon clips which personally I have no use for. But due to Elmer Keith, I blame him, many expect the 44 Spl. to be an extremely accurate round and more so than other rounds. That's not the case. It's as good at accuracy as the gun, the ammo and the shooter but no more, same as other rounds. As much as Keith and Skeeter Skelton loved it the 44 Spl is not endowed with any supernatural qualities.
The 44 Magnum is a good round and a great handgun hunting round but most folks can't shoot it with any consistency past 3 or 4 rounds. But it sells very well because many folks believe that it and the gun will endow them with the magic ability to kill a charging grizzly in it's tracks and kill a caribou at 100 yards with one shot over iron sights. At the same time the ammo is expensive and shooters complain that it hurts too much to practice shooting with regularity. Most shooters would be better off with the 357 Magnum, the 10 mm, 45 Colt or a good load and bullet with the 44 Spl. Or with the 44 mag from a lever gun or short carbine. Better to have a gun you can shoot well and use that than one, than rely on a pistol that you can't shoot well and hope that the spirit of Elmer Keith will save you when the time comes. So 44 mag? Overrated by a good many people.
The 223/5.56 This is a very good varmint round (descended from a varmint round) from a variety of rifles. It is also a good CQB round. Folks often forget why it was adopted by the U.S. military back in the day and attribute to it more power and ability than what it's good for. The 5.56 was developed around a light weight combat weapon as a round that could be used in both single fire, 3 round bursts and full auto and be controllable. Neither the 30-06 nor the 308 were useful in that function. The military needed those capabilities for combat in urban and jungle combat usually within 100 yards and less including hand to hand combat range. They also wanted it to be accurate out to 200 yards. They got that with the M16 and the 5.56 a 20 caliber 40 gr. bullet. The idea was not one shot one stop but a spray. Now a good many think it's a good deer or hog round. In an M4 it's very good at exactly what it was built for. But if you want to take deer or a hog at 100-150 yards bring a 7.62 or a 30/30 at least, you can brag some then. So .223? Overrated.