Had a normal M4 AEG with a 509mm barrel installed and an unported cylinder, I decided to make it a CQB gun by installing a shorter barrel that was 150mm long, the FPS suddenly dropped from around 400 fps to 130 (with .20's). I thought this was due to to uneven cylinder to barrel volume ratio. I calculated the volumes and drilled holes into the cylinder making a 2:1 cylinder to barrel volume ratio. I also added a Systema bucking and H-nub and the fps rose to 160 fps. I have great compression in the gearbox (double o-ring cylinder head, o-ring air nozzle, ect.) and so this is really puzzling me. Any suggestions to raise the abnormally low FPS?