I noticed that "CIP" chambering reamers are available for the .500 Nitro Express.

Does anyone know the difference, if any, between a CIP and a traditional .500 reamer?

CIP (Commision Internationale Permanente etc.) would be the certifying body for that cartridge, the only standard for that reamer.
Europeans standardized it. There is no other standard.
SAAMI does not certify it.
RIP does not certify it.


Riflecrank Internationale Permanente
Pet Wildcat Certifying Authority
NRA Life Benefactor and Beneficiary
https://home.nra.org/