Quoted: I recently purchased a 9MM upper and have been doing a good bit of shooting in it's new 9MM configuration. EVERYONE keeps telling me I'm gonna break my hammer and or trigger pin. Exactly why is that? I'd appreciate that bit of info. Thanks
Quoted: On an M16 that saw heavy 9mm usage I've had both hammer and trigger pins break, and I routinely replace the full set of lower fire-control springs every 5,000 rounds. They keep going and going and going..... 
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The pin breakage only seems to affect some guns, but it's worth putting in the hardened pins in ANY AR. They work fine with all calibers and have no drawbacks.
The problem is caused by a design flaw in the 9mm upper due to the use of what is basically the same upper for both calibers. The original 5.56 version utilizes a locking-bolt mechanism, but for 9mm they had to either heavily redesign the gas operating system or go to a simple blowback operation.
Like the AR15/M16 series, the 9mm upper was developed first for military applications and began as a full-auto design. But with the closed-bolt blowback system in the 9mm, rate of fire must be kept down to avoid out-of-battery ignition. Safe levels are around 1,200 RPM, but Colt (and the military) decided to keep it well under 1,000 RPM for safety reasons.
The way to reduce rate of fire is to increase bolt and buffer mass. Colt developed the heaviest buffers it could, and even went to hydraulic buffers (first used in 9mm guns), but it was still limited in the bolt-mass department: The 9mm bolt had to fit in the 5.56 receiver''s bolt channel, and there was no way to get enough mass to slow it down.
So instead, Colt redesigned the bottom of the bolt. While the 5.56 has a gentle ramp to "ease back" the hammer during recoil, for the 9mm Colt designed in a sharp corner that put much more pressure on the hammer/spring -- in effect, using the hammer spring as an additional "brake."
This succeeded in keeping the ROF down. Unfortunately, it also drives the hammer back HARD at the end of its travel, and it often ended the swing by hitting the disconnector -- again, HARD. This impact was transmitted through the disconnector to the trigger as well.
All that "hammering" eventually causes the hammer and trigger pins to break. So Colt came up with the hardened "white" pins that willl not break, and installed them in ARs and M16s that left the factory in 9mm.
Hope that long explanation helps.