double-r

A bent or missing hop spring will perhaps cause 15-20 fps loss, but I don't think that it could account for the loss in fps you are experiencing. Sadly I am not aware of any source for a "new" hop unit spring, and would suggest a spring from a cheap pen or maybe you could straighten out the one you have. Have you taken the upper gearbox apart yet? Loss in compression could be occurring at the cylinder head or on the piston "o" ring. Have you tried a compression test on the cylinder with your thumb? i.e assemble the cylinder head with cylinder and push forward the piston whilst holding your

Have experience of 3 ICS M4's recently (2 sportlines) and they were all shooting at least 330fps. Two were in and around 340-345fps with the last one at the 1j level (328fps with .2g). The 1j gun had the stock spring clipped down by a couple of coils, so I would say that as standard you should be looking at 330-345fps. I would check the cylinder head in the upper gearbox for sealing, and also ensure the hop unit is tight to the nozzle (make sure the little spring attached to the hop is nice and straight and bouncing back on to the nozzle/upper gearbox.) Good luck in any event.