You AR pistol is illegal here. That's why 30 round AR mags is kind of a grey area. Any mag that holds more than 10 rounds and can fit in a pistol is illegal. But AR pistols are illegal in the first place so it won't fit in any pistol hence they're allowed. Everybody turns a blind eye to it and I wouldn't even sweat it. Everybody at the range is using them, but only 5 rounds at a time.
You better read the gun regulations for Hawaii really good. Basically anything NFA is illegal and HPD (police) will confiscate it when you go to register. No SBR,SBS, suppressors, machine guns, etc. All the cool stuff is outlawed here. Next to Kali and NY, we have some of the shittiest regs.
It's not all doom and gloom though. Scohfield Barracks has a gun/fishing club open to all military. They shoot at the range on base where anything goes the last I heard. Mag dumps, rapid firing, shooting out to at least 1000 yds, etc. While us civilians only have Kokohead, the only public range on the island. 100 yd max distance, no rapid firing, 5 rounds in mag only. No 50 BMG allowed. The neighboring city is always trying to shut it down with lies about stray rounds hitting their roofs, hikers trespassing and walking into the line of fire. Anyways, not trying to bring you down but welcome to the islands and I hope you enjoy it here. Aloha!