I had a JR Carbine in .40S&W that took Glock magazines for a while before I thinned the herd a bit. I bought it used but is looked like it had barely been shot. Well after the first few rounds I knew why the original owner ditched it. Every other round or so would jam when being chambered. Upon closer inspection I found that the feed ramp had a very sharp edge and was digging into the brass. I pulled the barrel and rounded the edge on the feed ramp and after that it never had a issue again for several thousand rounds.
Only other thing I could possible pick about the rifle is that you have to remove the buffer tube to take the gun apart to do a thorough cleaning. I guess it has to do with the blowback design.