OK, now what does that mean?
It means the bullet is probably a .223 Remington or very close to it while the casing is much, much larger, almost 50 caliber size (20.4mm base). The Nitro express is actually closer but has a significantly larger bullet as it is meant to bring down big game.
https://www.commackschools.org/Downloads/Chapter16%20Firearms,%20Ballistics,%20and%20Impression%20Evidence.pdf
scroll down to page 12 and examine figure 16.1.21.
The two cartridges I originally cited have bullets in the approximate size but shell casings that are significantly smaller than the Whatzit.
I'm not having much luck finding a match in standard loads so I would have to assume this is an exotic. With that much powder behind it, the bullet will be traveling at extreme speeds. It would be counting on the speed/shockwave to do damage, not mass per se. This means a "soft" target, i.e. living tissue.
I used the diagram to look up specifics for the shells to narrow down the candidates. Unfortunately the photo lineup is based on LOA not diameter so I had to hunt and peck and then pull wikipedia pages to get specifics.
