Like everybody said, Koko head is pretty much the only place on Oahu. If your wife is army, I think there's a range at Schofield Barracks. It's a very restrictive environment. No concealed carry, unles you're the Lt. Govenor or the police chief's sister-in-law. Firearms must be registered within 3 days of arrival. No Class 3 AT ALL, you cannot bring them in state. Transporting must be locked, unloaded, ammo locked & seperated from gun. Want to buy? Pay first, go to HPD for permission, wait a week, then pick up gun.
There are some bills in the legislature this session to bring about concealed carry but I'll be shocked if they pass.