Round ball BP loads for ctg. guns were designed for shooting all the animals of chase, be they elephant, buffalo, rhino, etc.
: By the time smokeless loads came about, there were no more round ball loads that I know of as all the gauges had switched to conical shooting.
: Baker once said, that in having a slug mould made for his big double, destroyed it's killing/stopping power and led him into dangerous scrapes with animals. Prior to this, it had never failed to floor a charging elephant.
: That speaks quite highly for large bore balls.
: I've no experience with a .577. I did shoot a Wesley Richards 12 bore bar and wood a bit. Nice gun.
: My friend has a 6 bore double ball and shot gun with 34" or 36" tubes. It was a heavy beast, and H&H re-furbished it. When returned to will, it looked brand new. Absoltuely beautiful. They'd re-bored it to 5 bore, but other than that, made no other changes. It still had the rear sight lump, a flat-topped block with platinum line in the centre.
: I seem to recall it was regulated for 8 drams of powder with round ball, which is 218gr. with a 7 bore patched ball. This load would probably do about 1,200fps, maybe 1,300fps. Wil has shot only pellets from it, and did very well on 16 yard trap at the local club, 20 out of 25 and with no chokes, but 3 ounces of shot.