Yeah, they do that. But surprisingly, they don't have to scavenge for fully automatic weapons. They're everywhere. Everybody has one. Normal people have to be very rich and connected to obtain a full auto these days, but after the zombies come, it appears that every Bubba and his brother had a handful of 'em! And not just the little personal ones either. Belt-fed .50BMGs mounted to the back of their pickups even.

It may well be as successful as the Medusa M47. That revolver was chambered for .357 Magnum and could chamber and fire many rounds smaller than the .357 Magnum. With projectiles in the 9mm range the accuracy was acceptable according to reviews.
Having ammunition and a gun designed and zeroed for that ammo has also the advantage that you have a fairly good idea about the point of impact. My main hunting rifle does not even shoot all loads of the caliber it´s chambered for reasonably close to the point of aim for my hunting ammo.
If you are carrying a gun for a survival situation it might be useful to be able to hit something smaller than the proverbial broad side of a barn.

This sounds to me like the sort of versatile weapon that might be worth buying and storing away for any long term emergency, rather than carrying around in most situations.

Under normal conditions, including any localised or short term emergency or disaster, any standard type of rifle and plenty of ammo to suit might be better.

However in the very long term, even large stocks will eventually be consumed, but one might be able to find or barter ammunition of a calibre that does not fit your weapons.
Possession of a gun that accepts a wide range of ammunition could be literally a life saver.
Worth considering IMHO as a long term doom prep, rather than for routine hunting or defensive purposes. You might never need it, but that is true of many doom preps.

I like the notion of recycling and reloading found ammo which does require time, but which will likely result in a much better product. Besides, reloading is just about as much fun and interesting as shooting the rounds. I am sure stuffing powder X from a large caliber weapon into a 357 magnum would be very "interesting."

If your choice of firearm is of a caliber that is used by law enforcement, the U.S. military/NATO or a military caliber from the old Warsaw Pact nations you should be able to find ammo. Every firearm I own falls into what I would call a common caliber/chambering such as: .22 LR, 9mm, .357 Mag, .40SW, .45ACP, .308Win (7.62 NATO), 5.56 NATO, 30-30, 30-06 and 12 ga. There's lots of ammo for those calibers in personal stockpiles. I should add 7.62x39 which I don't have, but there should be lots of ammo floating around. My personal stockpile is in calibers used for competition and plinking.

Why have cylinders chambered for calibers you are unlikely to ever scrounge? In a rifle chambering you need a decent barrel or as has been said the muzzle flash will be outstanding -- but where did the bullet go? This is a one shot device -- one shot. It needs to be accurate and I do not see that attribute anywhere in this "rifle".

Be careful with recycling found ammo. Reloading Bullseye into a rifle casing could result in a disaster. If you cannot determine the type powder, this path may be one best left untaken. You could easily recycle 30-06 into a .308Win case, but taking shotgun or pistol ammo (relatively fast burning powder) and recycling into rifle ammo (usually much slower) could be explosive.

You mean you actually what to know what powder you are loading into a given cartridge?? What's the fun in that? Actually, I totally agree with your point. If I were ever at the point of pouring powder "X" into a case, I would go the other way round (put the powder in a large volume shell) and experiment very carefully.

It is ever so much easier and effective to keep a store of correct powder and primers for the firearms you possess.

Probably many (most?) of us already have at least one gun in each of these calibers anyway. So this new gimmick is pointless. They must be marketing it to the one-gun-solution type of buyer. But if I only had one gun, this thing certainly would not be it.

But there's good news on the horizon: If you really want one of these things, you can probably depend on the type of person who would actually buy one being the same type of person that would end up as road kill in the first wave of any attack. Then you could scavenge it off of them.

Putting the slow burning powder for rifles into pistol ammo might cause problems too. The case may not seal properly and blow back some gas. That is a common problem with CAS ammo too (I am working on a .45 Colt load that does not exceed the speed limits while sealing properly).
The only thing that seems to be universal is black powder. Full case and compressed slightly seems the way to go there.

I've seen reports that with Trail Boss powder (for handguns), you can get by with just filling the case. I certainly haven't tried that personally! I use and like Trail Boss, but I measure it, per standard reloading practice. And measured amounts don't fill the case most of the time. But they do take up significantly more case volume than other powders, sometimes to the point of almost filling the case in some calibers.

If I had to pick one handgun powder to use for everything, and wouldn't be allowed to keep anything else on hand, it would be W231. That one works with all the handgun cartridges I reload (but don't fill the case with it!)