Ha' I was under the other frame of mind. I have had experience with only one of these, a best WR, it had a bullet mold with various base and nose plugs that made a bullet of 270 to 340gr.
I had always sneered at the 577/500 mistakingly thinking it was just a stubby open point bullet setup.
This thread and the previous A. Henry 577 thread educated me on that. I have since found some reference to them handling quite a wide range of weights in regulation.

Could it be that all the load data, weights seem to use the similar 1800 fps-ish velocity ???
A 270 open point and a 570 hardened solid.....with some sort of regulation.....Oh' wow....very cool