own an M57 and it's a fine pistol. Quite honestly why would you want to convert it to a 9mm? It sounds like you have to mess around with the location of the round in the mag just to keep it from not jamming. There are plenty of perfectly good 9x19 pistols around. I'm not trying to be snarky about this. Just wondering.
Instead of asking why, how about asking why not? My 9x19 barrel cost me $40 and gives me the option to shoot another cartridge out of the M57. Not much of a down side and all it takes is hitting the mag a couple of times to line the rounds up.
I also go to a range that doesn't allow bottleneck pistol cartridges. Kind of dumb, but that's the rule. With a 9x19 barrel, I can shoot the M57 there.
I actually ordered two 9mm barrels when I bought mine and converted one to 9x23 Win. That's a whole 'nother story, though.