Do you have a Glock? Try to find ammunition that runs well in both.
There are some decent aftermarket parts available to smooth out the blowback operation. I use a heavier weight bolt and put a wider sleeve around the charging knob. I also did the screw replacement but that was not absolutely necessary. KTOG, mcarbo, tandemkross, and of course Kel Tec offer lots of parts and information on the fold up carbine.
I use PP for loading .40 S&W as well as .357 SIG. 155 gr and 125 gr bullets from Montana Gold respectively.
One of the best things about Glock mag PCCs like the Sub2K is the ability to pull the magazine from your pistol and run it in your carbine, IMO.
I was just reading the article in the American Rifleman on the CMMG ARs. The Radial Delayed Blowback sounds like an interesting way to deal with the need to slow the cycling of the straight blowback operation. Most of my PCCs cycling issues have been dealt with with heavier buffers.