pistols actually are meant to be limited to a muzzle 8ft/lbs if i recall correctly. so their fps limit should actually be lower.
but again, the frangible projectile usage throws the whole thing to pot.
the use of frangible ammunition is used to classify a paintball marker as an air rifle. however the characteristic of a frangible round is that it
cannot cause a penetrating injury. the major rule is the 12ft/lb for rifles and 8ft/lb for pistols and so paintballs fired under 330fps are legal.
a better way to understand it is airsoft guns. obviously the bb's are not frangible, but equally they are not designed to cause a penetrating injury. in addition they are also within the 12ft/lb limit because although they are fired at around 350fps if i remember right, the considerably smaller mass means it is well within the 12ft/lb limit.
this is just my understanding of how the law applies but do not take what i'm saying to be fact, it is just my opinion. i may be wrong about it but from what i have gathered over recent years this is how i view the law applying to paintball, airsoft and other firearms.