a TV programe called "SWAT shootout" mentioned a 9mm MP5 SMG with suppressor can slow down the bullet velocity... also heard handgun+silencer will end up with less velocity...
does this apply to modern airguns too?
Some moderators extend back over the barrel, wihch is ported to bleed gas into the moderator chamber, this would obviuosly have some effect on velocity.
The majority of moderators simply contain the gas emitted from the muzzle and have no effect on velocity.
Airgun moderators fall into the latter category.