Maui PD doesn't seem to want to look at my rifles when I register them, I've only been to the police department once. Does HPD really make you come in to physically register every single gun, and they actually measure them? Damn.
But this is sorta the concern I'd have. The ATF's method of measuring a barrel is well established, written down in clarification letters, if not administrative law. But if Hawaii decided that their 16 inches didn't include permanently-attached muzzle devices... Or that they measured from the "top of the balls" instead of the "base of the shaft" or whatever... I don't think it's on paper to say they couldn't, I imagine it'd be a court case but probably a winnable one. I'm willing to take the remote risk and sleep well knowing I'm at least complying with federal policy.
One can keep a 26" long gun shouldered while leaning sideways through a standard door frame or hallway, and it is really useful while keeping handy and positioning around inside of a vehicle too. I can take positions with a 26" gun that'd be physically impossible with a 30" gun. You can also fit it into a much smaller container, like a tool box.
I like bullpups. K&M, IWI, KelTec and DesertTech each make a 26"ish gun with 16"ish barrel in 308, and there are many more options in 5.56. Tavors/ x95's and AUG's have seen plenty of serious service; the layout is not a total evolutionary dead end.
The standard way to measure a rifle is from inside the barrel with the bolt forward. Stick a rod down it until it stops. HPD does confiscate SBR's like 1 or 2 a year, but i'm unsure of what barrel length. These are probably people who moved here from the mainland. Also IDK if they were charged with anything since it was confiscated at HPD when they tried to register it.
That's good that MPD doesn't want to look at your rifles. Yukutake, etal vs. Conners (HI AG at the time), lawsuit by HIFICO allows online reg if you buy the gun from a HI FFL. There is no need to bring these guns to the police station anymore. But private party or out of state, then you must bring the gun to the police station. Prior to this lawsuit settlement, all guns had to be brough into the police stations in your county. This was changed like 1 or 2 years ago.
I did look into pullpups, but decided against them.
1) The Tavor felt very off balance. Compare this to an AUG, Keltec RDB or Desert Tech MDR. Then the gen 1 Tavors, I like their mag release compared to the gen 2's.
2) If I went with Aug, I need all new mags as it doesn't accept Pmags.
3) The price of bullpups are often double what you can get a basic AR for.
4) The cheaper one, like the Keltec, felt cheap and not as sturdy. I never looked into it's reliability, but it is a Keltec after all so....
5) Then the IDF switched from the Tavor back to the M4. So this combat tested army did so for a reason.