I’ve just recently bought an Umbrella Prometheus 6.03 EG inner barrel with R-hop. I dropped it into my gun with a G&G CM16 Raider receiver with a Polarstar Fusion Engine installed. The first 300 shots were firing perfectly, even on full auto. The following day I went to go play on a field, and it instantly started jamming. The bb’s were being stuck right on the lips of the bucking. The bucking doesn’t seem to be torn either. I’m using Valken .30 BB’s. I haven’t tried shooting with other BB’s because Umbrella has told me to try and use the same BB’s throughout the ‘break-in period.’