I bought this as the K3P PDW (pre-factory paddle release) and had a well known HK "smith" type do the trigger, add the paddle mag release and weld on a German steel brass deflector. It was SBR'd and then test fired only to find out that the brass deflector had been welded about 3/8" too far forward and was bouncing ejected brass back into the ejection port.
It took a lot of grinding on the brass deflector and milling it back to almost just the welds to get it to eject properly. But then I found the other issue. The bottom front edge of the bolt was beveled so that the rounds on the right side of the magazine were getting deflected down and not chambering, i.e. failure to feed. Who at PTR thought this was a good idea?
Thankfully I have a PTR GI that didn't have the bevel on the bottom of the bolt and swapped bolts for the function test.