Even if it was assembled into a grenade launcher, it is not on the list of banned items under HRS 134-8(a).
HRS 134-8(a)
The manufacture, possession, sale, barter, trade, gift, transfer, or acquisition of any of the following is prohibited: assault pistols, except as provided by section 134-4(e); automatic firearms; rifles with barrel lengths less than sixteen inches; shotguns with barrel lengths less than eighteen inches; cannons; mufflers, silencers, or devices for deadening or muffling the sound of discharged firearms; hand grenades, dynamite, blasting caps, bombs, or bombshells, or other explosives; or any type of ammunition or any projectile component thereof coated with teflon or any other similar coating designed primarily to enhance its capability to penetrate metal or pierce protective armor; and any type of ammunition or any projectile component thereof designed or intended to explode or segment upon impact with its target.
It could be built into a Hawaii legal firearm.
Did you get blocked at the registration phase or the permit to acquire phase? If registration, try the online portal.
Are you going handgun or rifle/shotgun receiver registration for it?
Thank you for being the guinea pig for this. I wanted one, but the idea of either needing to have a 16 or 18 inch barrel made or having to shoot it pistol grip only did not appeal to me.