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psalms144.1 I second this. My first AR-9 was a 5.5 inch build with a quad rail handguard. Adding sights and a light left little room for anything other than a magwell hold, and I'm a little guy. It might have been better with a VFG and today's slim M-LOK handguards and accessories.
What do you plan to do with it? Home defense, range toy, competition, trainer for a centerfire SBR? My current build is for the latter three, and I will be using an 8.5" CMMG Radial Delayed Blowback barrel and bolt on a standard SBRed lower using their 9mm adapters for PMags. It's a little pricier than a standard blowback build, but it's smoother shooting, has a bolt carrier that can be tuned, and I can train with full size AR mags. If I were to get a PCC for serious use in the smallest package available, I'd probably get an MP5K clone or B&T APC9K if a folding stock is important, or I'd consider the Glock mag version of the CMMG Banshee with the 5" barrel if you want the standard AR controls and don't care about a folding stock.