In April 2017 I built a 7.62x39 AR using a Palmetto State Arsenal 18" 7.62x39 mid-length complete upper, a Polymer80 lower and a lower parts kit. I purchased 5 C Products 10 round magazines, a scope and 200 rounds of Wolf Military Classic ammo. The rifle functioned beautifully right from the start. Since then I have fired an additional 5000 rounds of the same ammo and then it stopped cycling. Prior to it failing to cycle it would occasionally fail to feed the second to last round in the magazine but otherwise the gun functioned fine with good accuracy. I also have AR's in 5.56 and 350 Legend. I swapped lowers from them to the 7.62x39 upper but it still failed the last round in the magazine test. The 5.56 and 350 Legend uppers functioned fine with the 7.62x39 lower. The spent cases mostly ejected in the 3 to 4:30 sector. I installed an H1 buffer and new flat wire spring without success. Convinced that the problem lay in the upper, I removed the gas block and gas tube, plugged the hole in the gas block and filled the tube with KG Systems Carbon Remover and let it soak for 30 minutes. The solvent was clear when the tube was filled but was deep blue with tiny black particles when drained. I drilled out the barrel gas port from 1/8" to 9/64". The difference in the OD of the gas tube and the ID of the gas key is 0.003"/ 0.004". The hole in the bolt carrier was slightly blocked by deposits. I drilled out the slanted hole in the gas key with a 1/8" bit. The bolt rings passed the test in which the extended bolt supports the BCG. It didn't collapse even when a 12' tape measure was placed on top. After all this it still did not cycle properly. I even removed all the weights from the buffer with no success. I don't know which part of the upper is at fault and with this present hysteria it is hard to find parts in stock.