portfiner,
i think i can give you some general ideas about what you are asking for.
First you have to decide which band and shaft configuration you will use before deciding on barelll dimension.
Since i am quite familiar the diving conditions you told and have similar type gun. I will advise you either 6mm shaft with 16mm bands or 6.25mm with OMER power 18mm. Difference between 6.5 and 6.25 is really huge, you can feel the difference at recoil, shaft speed, and reaction of the fish after speared.
Well, you already know you need oval barell shape for maximum side tracking and recoil absobtion.
if you will use seperate handle and muzzle you will also use some branded reel.
Look, i will advise you what i had used and liked.
For muzzle, in my opinion excalibur 2000 is the best muzzle design i have seen in flesh an used up to date, especially if you modify it the way i did. That way you get all of the pluses of the open muzzle and screw in muzzle, and belive me it makes a lot of difference while tracking and shooting fish. Muzzle is one of the major "accuracy disturber" if you set something wrong.
I liked OMER'S Alluminium handle at best. Its the preciest and smoothest i ever shoot, what i minute, you supposed to have one isn't it?Plastic Excalibur handle will do the job nice too.
In that case reel will be alluminium omer, if you use it with alluminium handle it will be back heavy but the balance of the handle never effect the nose behaviour at all. If you use it with excalibur handle it will be slightly more bouyant. I am shooting with one of my friends 90cm woodie, which has alluminium handle and allu 30 reel and belive me it track and shoots as well as others. I even like the feel of it.
Now the barell, if you will not consider to putting on excalibur muzzle which i strongly recommend you to put one, obviously you better to choose thin nose in that case, since the fishes and environment you are going to hunt will demand fash aiming and shooting. Because of the shaft and band configuration recoil will be minimal so you don't need to worry about nose lift much.
Now i will give you some universalized barell sizes (for that kind of wood guns of course!!!), small cuttlefish bone shape will be perfect in my opinion, so with 5-5.5cm at the center, 3cm at the front, and 3.5-4cm at the back.
I thnik that all i want to say :hmm
