Ewwwwwww 16 inch guns.
Honestly, its all in the upper. The allure to a KAC gun and its reliability are basically all in the fit/smoothness of the gun and the bolt. A lot of people say its the bolt itself, but outside of the actual carrier, all of the parts are redesigned and different to allow for better operation that puts significantly less stress on the individual parts. The E3 bolt is different (and proprietary), the firing pin is stepped and profiled differently, the cam pin is stepped and profiled differently. I wouldn't be surprised if the carrier had something different about it as well.
It's a carbine running a midlength gas system. Add a MAMS into that and there is basically zero recoil.
The barrel and gas system are designed where basically KAC tells you that if there aren't 30k rounds on the barrel, there's no problems with it.
If shit is getting this rare and pricey, I'd just sell a few of the 16" complete uppers I have,