My VFC SCAR was very sensitive to hopup rubber/nub. Too hard or too much and it would jam till it had a few BBs in the chamber and the bloop four out at once. I found the Guarder clear or the black Systema with the respective nubs worked best.
I also found it to be very tough on batteries. My only guess on that is that the hidden contact>hinge/gearbox>lower connections had a ton of resistance. I got rid of the fuse and ran a 9.9v 20C 1400mah LIFE Battery that would fit in the stock. That would run for a days worth of gaming.
Airsoft, where nothing is hurt but feelings.