I've had my combat machine (non blowback w/ris) for just over 3 years and haven't had any problems yet. However, i noticed my fps is dropping. I've never upgraded this gun but have upgraded many other similar m4s. i remember when i first got the gun, it shot at about 360fps. Now, its down to 280 and less than 60 feet of range. Could it be the spring? the air nozzle's o-rings gone bad? or a bad piston head or cylinder head seal? I would also like to upgrade the gun to 420 on the dot consistently and reliably. I don't want my combat machine to go from workhorse to racehorse. I still want it reliable, but i want the range and fps increased. Also, I have the cqb barrel version of the combat machine. However, i installed a mock silencer that extended the barrel to the length of a full size. Any suggestions?