VFC puts a spring inside their cans between the foam and the BB path to tame any unruly foam.

They aren't the only ones, Remove the foam and take the end cap off your silencer if you want your gun to sound "Different"

Well... when you're firing full auto on an AEG, the bb's following your first few shots will be affected by the turbulent air left in the first bb's wake (that is bouncing around in the suppressor tube), affecting accuracy slightly. So in theory, suppressors fitted on AEGs firing on semi auto, or bolt actions wouldn't be affected. But regardless of application, as stated, the bb should not be affected fps-wise whatsoever by a suppressor if attached correctly.