How much clearance is necessary at the rear of a chamber to accomodate a cartridge? How much clearance would be needed for a cartridge of a hypothetical .500 inch at the head? Would three thousanths be too little, ten thousanths too much? I am looking for a number to represent the chamber dimension for this case head that would allow for sure and positive chambering and be within "standard" chamber specs.

Do you mean how much larger should the reamer be than the nominal case drawing?

your case drawing will define how the maker wants to make the reamer

.003 is what some reamer makers do for DGR rounds... that's pretty much huge.. 3 thou wide and 3 thou long

jeffe