Re: .308 Case with a .223 bullet
Sounds like the Accelerator round. Already been talked to death a few times on this site. Marginally effective attempt to get blazing speed out of a .22 caliber jacketed bullet. Made in .308, .30-'06 and .30-30 chamberings.
Accuracy is generally marginal, unacceptable for match, and maybe also for varmint applications. If you want to smack the sh*t out of something relatively close, it works; but further out it'll probably going to miss.
It looks simple, but just about everything contained in that case is a special purpose, one-off, proprietary component, and is not duplicable by any known combination of standard handloading components.
Have fun, then be prepared to get over it and back to the real world asap. Won't hurt your gun, and that's about the only real good news about it.
Greg