I had a chance to take apart a 2010 ICS M4 a few weeks ago.
ICS IS FREAKING TERRIBLE.
They're using technology in their airsoft guns that's 10 years old. In fact, I'm pretty sure they just bought all of tokyo marui's old plastic injection dies lol
If you want any kind of half decent performance out of an ICS, you have to replace the cylinder head, piston head and air nozzle right out of the box. They have almost no compression.
I got an extra 80fps just by changing those parts.