The problem with shooting 9mm in the .357 NMBH is that the throats of both my .357 and 9mm cylinders are .358”. This is fine for the .357” bullets of the .357, but it is a little oversized for the .355” bullets typical of 9mm. That said, my .357 NMBH is very accurate with the .357 cylinder, and nearly as accurate with the 9mm cylinder and CCI Blazer elchepo 9mm ammo. I didn’t expect that level of accuracy with the 9mm cylinder and was very pleasantly surprised.
I also have a couple of .45 Colt NMBHs with .45 ACP cylinders. The .45 Colt chamber throats were very tight. When the throats were opened to .4525”, things got much better. One of the .45 ACP cylinders was also very tight. I opened it with the same reamer. It is now very accurate, too. For range work, I am usually using the .45 ACP cylinder.