Is there any shops out there doing prefits in 300 Norma imp? I like to do a prefit before I spend the money to have a full custom built. I have not been able to find any of the typical shops doing an improved version. Just the regular chamber.
I doubt if anybody will ever offer a pre-fit in an improved chamber of any parent cartridge.
You could buy a standard chambered pre-fit and chamber it to NMI.