I don't know why. My last one was a brass frame by ASM and it shoots fine. The only brass-related disadvantage I can see is possible glare from the brass sight channel. But where I have shot it I haven't had a chance to evaluate that. Maybe you might make a case for harder to keep the brass clean and shiny. But IMO that's just a small price to pay for its good looks:
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The only real disadvantage for me to THIS particular revolver is its standard grip frame, which is somewhat cumbersome for me to properly grip and properly position my trigger finger. I prefer the large-frame version, which provides a little more distance to the trigger.