I've been playing around with a 9mm cylinder in my New Model Blackhawk, with 6.5" barrel, and Tasco 2X20 scope. This is not a convertible, as I picked up the extra cylinder from a fellow on the Ruger forum.
The Blackhawk is superbly accurate with .357 HBWC loads (sub one inch groups at fifty feet), but is very selective about 9mm loads.
Hornady 9mm, 100gr FMJ (not XTP) bullets loaded to 1300 to 1400 fps, will shoot one inch groups for me at fifty feet, while lead bullets shoot groups at least double that size, and often 3" or larger.
These Hornady bullets are no longer made, but can be found at gun shows, so I'm collecting all I can find. They have exposed lead at the base, measure .356" diameter (which may explain why they shoot so well while other 9mm bullets do not), and have a semi-hollow base.
A true convertible may be more accurate, but mine is certainly acceptable for plinking with standard 9mm ammo, and has been very accurate with the special reloads mentioned above.
Bill