I think you asked about the 9mm Colt 1991, right. Well Colt uses an unramped barrel in their 9mm. I believe. I not sure about the dia. of the barrel but I think they are full sized then reamed to 9mm instead of .45. I also think they weight more than a .45. Not sure exactly about this but I'm sure someone will come along and have the truth. I've heard they shoo fine but no first hand experince.
Cyric