I designed a supressor years ago for my KSC M11A1 that actually increased the velocity by 30fps and also made the gun super quiet, you could really only hear the clacking of the bolt. Inner barrel in the supressor helped, as well as only having the last half of it perforated that allowed the gas to expand in the irregular foam baffles I installed.
And no, didn't get any 100km kills with it.......