..as I wrote " new barrels, modern steel". It wouldn´t become too unrealistic...and as I wrote
"1900-2000 ft/sec" which means "1900ft/sec" would also be fine.
If I choose to have a new set of barrels done on my 12bore Rigby the chamberwall thickness will become .289" at the breech (casehead). My Rigby is done in 1903.
I know the originals were in the 1800-1900 ft/sec with a 440grain bullet. If I get/find an original I would natually remain within the original regulation "bounderies".

How much does your .577/500 magnum measure at the breech(casehead)?.