Thanks so much for your answers!

450's definitely seem to be the easiest to find, although there have been quite a few 500's around as well (I missed out on an amazing H&H from 1891... gutted).

Tinker: here you on the slugging the bores, and do have a gunsmith here in the UK ready to "inspect" whatever I find, before buying. BUT, not a specialist in this kind of thing. If anyone knows of someone in the South of the UK who would have good/great knowledge, would be very greatly appreciative of any names!

Daryl: I get your point, but not having any experience with the .577, won't the reciol be on the "hefty" side? Most of the shot are between 5 - 40 meters (although they are moving at quite a good clip...). I am mindful of reciol, to have a quick second shot and not completely lose sight-picture (they tend to run in groups of 3 - 10 animals, but quite packed together).

Thanks again!