I would NOT recommend a 55 gr .223 bullet for ANY big game animal.I suspect that with a 55gr FMJ from a 223 using the same shot you'd get the same result!
I would not recommend it either. But that does not mean a well placed 55gr solid won't kill instantly if placed right.I would NOT recommend a 55 gr .223 bullet for ANY big game animal.
That said, a US Forest Service LEO, a Sheriff Deputy, and I once shot a charging sow black bear by each of us firing a single shot simultaneously. I'm sot sure what the LEO shot, the Deputy shot a shotgun, and I shot the LEO's .223 with whatever bullets he had in it. She died instantly.
I would not hesitate to hunt black bears with any deer hunting cartridge shooting expanding hunting bullets. I have several friends who have killed dozens on Montana black bears with their .22-250s.
The two black bears that I've shot while hunting them (spot and stalk) each died almost instantly from a single shot, one from a 220 grain cast lead bullet from my 1911 .45 acp and the other from a cast 250 grain lead bullet from my Ruger SBH .44 magnum.