I am not certain about Riley's arrows being ineffective. At least against standard zombs they are quite lethal; in the arena she killed one zomb after another. And CJ being able to take down standard zombs with a blade most recently (i.e. December 2009) proves that the standard zombs don't mutate in the way that their skins grow thicker and thicker.
The only two zombs that did not care were Ink and the Behemoth. We have neither evidence nor clues or hints about the skin of Smart Ones or Randy, how ell they perform against any bullet or arrow. So I think Riley's arrows could be any of the three possible outcomes: effective, ineffective, partially effective (whatever that might mean). As for the tranqs: interesting. What are tranqs? Are these those big guns which veterinairians use to take down elephants in order to take blood samples or to prepare them for a helicopter-transportation to somewhere else???

Best wishes!
Liam