They certainly can hold you for 48 hours before charging you formally, but they still have to be able to articulate their suspicion that you've committed a crime. Failure to do so will most certainly leave the department open to a lawsuit, although people rarely take them to task for it. It's been my experience that they will threaten you with arrest just to get you to "cooperate", most people don't know any better so it works for them most of the time. It generally upsets them when you know your rights and exercise them, but remember that it's very important to know if you're on the right side of the law before you start giving them a hard time. Not having your firearms there when registering is a big no-no on your part, but I can't see them threatening you with arrest unless you were being unreasonable. Most of the guys behind the desk are pretty cool and I try not to blame them for the shitty policies they have to enforce. Same thing with cops out on the street, I was present at one incident in Kapiolani park where some friends were having a GTG and without thinking had set up a table with about 10 bottles of hard liquor out in the open. A couple of bicycle cops rolled up and "reminded" us that it's illegal to drink in the park. In the end, because we were cool with them and didn't act like assholes they just told us to put it out of sight and left us alone. Your attitude plays just as important a part as theirs.