Interesting. I figured the longer barrel would not really incur any disadvantage in this usage, and might offer a slight (slight!) accuracy advantage.
?
Actually, 12 to 14 inches is enough to obtain peak accuracy in 9mm. It's a pistol bullet that doesn't have good ballistics to begin with. The longer 16 inch barrel does obtain peak velocity. Any longer than that, and velocity may suffer with some loads. There isn't much powder to burn in a 9mm.