Mauser

"I can make it jam occassionally in my CZ if I use 350gn round nose."

Not a valid comparison, the gun was designed for the standard bullets like 235gn, 270gn and 300gn.

Going to a 350gn or non standard bullet shape - ie the flat nose can't be compared.

"My 458 with 510gn round nose is at the other end of the spectrum - jams 50% of the time. "

What 458 ?

Winchester Mod 70 or CZ ?

Known problem because Win came out with a different shape bullet (510gn) and a lot of people had to modify the feed ramp of other guns to get them to feed normal 500gn bullets (and 400gn). And vice versa, some Win Mod 70's had to be modded to suit other bullets.