How about a ring shaped rim that slides down the RUM case and expands to .550 inside diameter and then snaps into the existing extractor groove to provide a .600 rim? These would single stack in a slanted magazine and feed at least as smoothly as the 303 and 30-40 Krag.

This straight case would hold a .510 bullet with a neck diameter of .540. A 600 gr bullet would have a sectional density of .330 and make 2400 fps for 7800 FPE in an H&H length action.

??? 50 -110, 510 wells (bigger case) turn the rim down on a 500 nitro

jeffe

500 Nitro Express. 510 Nitro Express. Without all the extra work.

Ray

My 510HE, my rimmed wildcat, case 3.55 in
long, straight case for falling block and break actions.Made by straightening 475 #2 brass,
with simple expander.And run though a sizer
die... 600 gr at close to
3000 in a bolt test rifle.Throat good for
600 gr round nose bullets, which allows use of
bmg spitzer bullets.Ed

SSK .510 Whisper ....