Depends on which version you have. The 300 Norma “Improved” is not standardized by any means. Seen the NMI with shoulders varying from 33 degrees to 40 degrees and everything in between. Neck diameters from .337” to .346” and everything in between. Some have the body taper taken out, some don’t.
@Alex Wheeler has dies for his own version of the 300 NMI but not sure if they will work for your chamber design. Can talk to him about your chamber dimensions and see.
Other option, which is what I did, is have a Wilson seater blank reamed out with your chamber reamer. That’s the only real guarantee for a perfect fit