Disregarding the bill entirely...
1) If you register a firearm that can fire multiple calibers, under current policy, what are you supposed to put on the registration form? Is an AR "223" or is it "multi"?
2) If, under current policy, you changed a gun like a Glock from one caliber to another, so the registration says "10mm" but it is now chambered in .40, are you supposed to tell the PD? Could you get in trouble for that?
3) As things are now, does any PD in Hawaii allow joint registration of long guns under spouses' names? Do they all allow joint registration of pistols?
You want to disregard the subject of your thread? Okay -- it's your thread.
In my experience, as long as you don't lie on the application, you can put whatever you want on it.
I've registered a rifle that was sold with 3 different caliber barrels: .22LR, .22WM and .17HMR. When I took it to the police station for registration, I purposely installed the .17HMR barrel, because I knew the previous owner registered it as .22LR. Nobody cared. As long as the rifle matches the registration, less work for them. I also "neglected" to include the other 2 calibers on the form. For all they know, I could have purchased more barrels after the rifle, and I can also sell them and only keep the one I like. They don't track non-firearms, which those barrels are.
You can buy conversion kits for pistols like the P320. Multiple calibers, multiple sizes, multiple colors, etc. Just drop in your modular trigger group, and your gun just had caliber-change surgery! The conversion kits are not firearms, so they aren't required to be registered. Whatever configuration you intend to use the most is the best course normally, but it's your choice.
The cynic would argue that capability can interfere with law enforcement and investigatory aspects of registration. If HPD is looking for all .223/5.56 caliber rifle owners to scan for suspects, and you registered yours as 300 Blackout, it would take a person knowledgable in firearms to add that, and all other calibers an AR-15 might use in order to produce the proper reports. That's not your concern.
The realist would point out that registrations are rarely -- if ever -- the reason all solved gun crimes are closed. So, the point above is moot.
Answer to both #1 and #2, based on the above, would be:
whichever caliber the firearm is presently configured to use ...
unless you want to change the law and require re-registration every time you change calibers.Answer #3 is "yes," for any legal firearm. Both applicants must complete the minimum-requirements to register a firearm on their own. Also, if you bought the rifle under your own name a year ago and didn't renew your long gun permit, but now want to simply "add" your wife's name to it, sorry. You have to go through the same steps again yourself simultaneously with your wife to change the registration as if it's a transfer. With the changes in training classes and mental health evals, it's a bigger burden for the current owner than before.
HPD seems to believe only married couples are allowed, since they ask for your marriage license, but i don't think that's a law -- just their belief/policy. They like to make shit up. Do they allow for unmarried couples who live together for decades and have kids together? Do they allow for other family members (parents and their children) to co-own a firearm? We never know what they allow until someone posts what HPD did or did not allow. And as usual, YMMV depending on who's assisting you when you show up.
FYI, the P320 modularity was developed for the most part to market in countries that have a limit on the number of firearms one can own. Registering one firearm with the capability of having a .22LR for practice (especially with new shooters and to save ammo costs), .357 Magnum full-size for home defense or when hunting, and 9mm compact for concealed carry would be a strong selling point in those areas.