Op, I bought a complete SR-15, shot it while I was waiting on the tax stamp for the lower. Once that came in six months or so later, I sold the upper and purchased my 12.5” upper through the EE. It’s a Vltor MUR with a KAC URX quad rail and stainless Noveske barrel. It’s a tank, but runs reliable and shoots super smooth. I would have swapped the rail years ago, but Knights uses a proprietary barrel nut AND wrench. The wrench cost over $100, so I decided it was “stuck” to the upper receiver! 
. I’m glad I kept it because I’ve determined I prefer the quad rail feel over the slimmer, flat tubular rails. I recently sold a BCM KMR off my 10.5” and replaced that with a Centurion quad rail. Noticeably heavier than the KMR, but shoots so much better with the little bit of added weight.
I’d suggest a Vltor MUR upper for your KAC lower. The fit is tight. The lines of the MUR look really good (to me) and it’s a quality, “thick-walled” forged piece to compliment your KAC lower. I believe there’s one for sale on the EE. You should buy it before I do!
Tuning was explained by another poster. SBR’s can be unreliable at first. I swapped the buffer spring and bought several different buffer weights to find the combo that cycled reliably and gave me the feedback I was looking for. I assume it’s mostly driven by the amount of gas coming from your gas block. Adjustable gas blocks are one way to address that issue. They cost more and reviews are pretty mixed. Neither of my SBR’s use adjustable blocks and both run fine suppressed or unsuppressed.
Again, congrats on your purchase and good luck with your build!
Here in America we are descended in spirit from revolutionists and rebels - men and women who dare to dissent from accepted doctrine. ~Dwight D. Eisenhower, address, Columbia University, 31 May 1954
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Originally Posted by oldbrowndog
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