There are no refs so calling hits is based on honour.
Generally, the effective range is a tad bit better than paintball, but don't expect crazy accuracy or range (as I did when I first started). Of course, this varies greatly depending on what kind of weapon you are using. ie. AEG, bolt action, or gas pistol.
The bb hits also hurt a lot LESS than paintball (IMO). The only welts that I got playing AS were from bare skin shots as opposed to the angry circular ones from PB.
Hope that helps.