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If I may... when converting the G23 to shoot 9mm Glock told me the safest way is to buy a glock 19. SECONDLY: if you wish to deal with just one receiver the best thing to do is buy a G19 upper. Then all you need to is take down the slide upper, slide on the G19 upper, change the mags and you are up and running in just a few seconds without going third party anything... NOW having said that if you notice on the trigger housing, the extractor is straight on the 9mm and has a little curve on the .40 (I don't like the .40 caliber by the way). This bend on the extractor will send the spent cartridges in a different directions. A purist told me that if you switch uppers you also need to swap out the trigger housing assembly to match the caliber. This takes a little longer as you can tell but certainly do-able. My question is why have a .40 at all? I know some say it is a more potent caliber and has a greater coefficient of energy dump. What they miss however is that it is not the caliber but the bullet sitting on top of the cartridge. Bullet technology has been improving a lot and there are manufacturers that have increased the lethality of the 9mm to surpass that of the .45. How? Check out LEHIGH DEFENSE. These are solid copper bullets CNC machined to be quite nasty. I am waiting for Socialist-Fascists to sound the call to ban them. There is another cartridge technology which also has solid copper tips but each bullet divides into three parts tethered and makes a heck of a mess as it impacts the target in three connected impact points.
Guess I can't post a link yet.. New Ammo Technology For Self Defense, ATF Approved ! weaponseducation on youtube