I have a JR Carbine in .40S&W. I picked it used at my LGS for $500 out the door. I kind of suspected that there may have been an issue with it even before I bought it because the LSG had only been selling them for a few weeks and already heres a used one.
First time out I discovered the problem. Every few rounds it would jam one in the chamber at a 45 degree angle. When I removed the jamed cartridges they would have a deep cut in the middle of the case. Closer inspection revelaed that the feed ramp on the barrel had a very sharp leading edge. I pulled the barrel, rounded the sharp edge with a file then polished the feed ramp with a dremel. Now it feeds flawlessly no matter how fast I pull the trigger.
I have an EoTech with a 3x magnafier on it and the thing is a hoot to shoot. I have two Glocks in .40 S&W, a G27 and a G35 so the JRC is an wonderful addition to my .40's.