Please use caution! ATF defines pistol or handgun as begin designed to be utilized with one hand.. (which is absolutely crazy as every proper, non "sport" handgun grip style is two handed)
Having a forward grip on a pistol, or just a place easy to hold, may be construed as manufacturing a AOW, which is an NFA items not not legal in HI. Come to think of it, I know that short barreled, suppressors and full auto is banned by name in the HRS 134, I don't know if AOWs are.. This might be a totally legal option but would still required a $200 tax stamp… Im going to look into it.
plagiarized from internet: (but I agree with what it says)
ATF has long held that by installing a vertical fore grip on a handgun, the handgun is no longer designed to be held and fired by the use of a single hand. Therefore, if individuals install a vertical fore grip on a handgun, they are “making” a firearm requiring registration with ATF’s NFA Branch. Making an unregistered “AOW” is punishable by a fine and 10 years’ imprisonment. Additionally, possession of an unregistered “AOW” is also punishable by fine and 10 years’ imprisonment.
To lawfully add a vertical fore grip to a handgun, a person must make an appropriate application on ATF Form 1, “Application to Make and Register a Firearm.” The applicant must submit the completed form, along with a fingerprint card bearing the applicant’s fingerprints; a photograph; and $200.00. The application will be reviewed by the NFA Branch. If the applicant is not prohibited from possessing a firearm under Federal, State, or local law, and possession of an “AOW” is not prohibited in the applicant’s State of residence, the form will be approved. Only then may the person add a vertical fore grip to the designated handgun.