Kart 9mm Barrels

First topic message reminder :

I rebarrelled a 9mm ppc pistol recently for a top level shooter. We changed it over from 1:32 twist KKM to 1:16 twist Kart. Ransom rested today and had good session. Had 2 different brands of ammo at 1" @ 50 yds; Atlanta Arms 115 gr and Hornady 115 gr XTP. 
KKM barrel has short throat similar to rifle. Kart has typical SAAMI style throat. Kart has less felt recoil. Is it because throat (more jump and gas blow by), bore diameter (KKM probably 0.001" tighter)? Both barrels similarly fit. Kart handled lighter loads well from hand. KKM didn't like reduced loads. Based on today's rest results and shooting from hand Kart looks darn good. I'm going to be busy rebarrelling pistols in near future.
Jon

Jerry, thanks for the background. I'll stop whining about it now.

BACK TO THE TOPIC OF THE 9MM KART BARRELS.  I HAVE A ROCK RIVER 9MM  5" WITH A KART BARREL. IT SHOOTS ABOUT AN INCH AT 50 YDS WITH BOTH 147 AND 115 XTP BULLETS. I HAVE SOME WITH THE KKM 1/32 BARREL AND THEY SHOOT JUST AS GOOD WITH THE 115 &124 GR . BUT THE 147 IS ANOTHER STORY. THE 1/32 WITH THE 147GR ARE AWFUL.

THAT BEING SAID AS A PPC SHOOTER ALSO. I HAVE SWITCHED BACK FROM THE 9MM OPEN GUN TO THE .45ACP. WHY ?? I'VE GOTTEN OLDER AND I CAN'T SEE THE HOLES AT 50YDS TO CHECK MY GROUP PLACEMENT WITH THE 9MM. NOW WITH THE .45 I CAN SEE THOSE AND MAKE THE ADJUSTMENT EARLIER. JP

Jon Eulette wrote:9mm is most common caliber. Most PD's supply it. I have built him a matching 45 pistol, we are still seeing what works best.  Since he is supported with 9mm the 9 is good option. I've shot both pistols side by side and 9mm is recoiling slightly less than the 45. His 45 barrel is picky and needs a little more velocity to group well at 50 yds. My similar pistol will group with .4 less powder which recoils less than the 9 does. We're spending quite a bit of time trying to see whats going to be the winning combo. 
Jon

Thanks for the detailed response Jon.