I have the .357/9mm version. The reason for the inaccuracy is that the .357 and .38 bullets are .357" in diameter and the barrel reflects that. The 9mm round is .355" and therefore does not grasp the rifling as well. Whether or not your gun shoots the 9mm well comes down to the actual diameter of your barrel. Don't expect great things. On the other hand I've had no problem with "keyholing" either. For hunting or serious target shooting, stick with the .38/.357 cylinder. If you're plinking at tin cans, then the 9mm is fine.
Allan - usually I use the 9mm cylinder to get the idea across to a newbie about how the weight of the gun tames recoil (i.e. shoot a light 9mm pistol and then the Blackhawk)