What I would suggest you do is to find another Model 66 cylinder and have that fitted to your gun and have it machined for 9 MM. TK Custom did that for me. You keep the integrity of your Model 66, yet have the versatility of the 9 MM extra cylinder. But if you just want your existing cylinder machined for 9 MM, TK Custom would be the way to go. Their work is excellent and the turn around time is very fast.