Thread: Light tuna gun
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Old 05-08-2012, 01:47 AM   #7
Ric.Fallu
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Join Date: Nov 2011
Location: Victoria Australia
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Re: Light tuna gun

I went down to the water today to ballast and balance the gun. Fifteen degree air and fifteen degree water. There was a stiff northerly chopping the surface too. It was bloody chilly but I got the job done. At least there wasn’t a horde of families looking at the weapon with disapproval.

I needed to add 1.280 kilogram of ballast. Seems a lot, but pine is light wood (SG around 0.3 or 0.4). The in-water balance point for the ballast was 9200 millimetres (about 3 foot) from the tip of the muzzle.

The gun, including spear and rubber, massed at 4 kilograms so, once ballast is added, it will be about 5.28 kg. Converting this to imperial measure, that’s about 12 ½ pounds.

In the previous thread on inertia needed for powerful guns, Phil suggested that the appropriate mass was likely to be between 3 and 4 pounds per rubber. Death Descending is a four rubber gun (so that means it has just over 3 lb per rubber) and just gets into Phil’s category, but might be a tad light-on.

Of course, this pre-supposes that the rule of thumb given by Phil includes the mass of the spear. If not, the spear and slip tip are about 700 grams and this would make the gun just a little too light. (If Phil reads this, I would be interested in his comment, should he care to give it.)

I came back home, chopped mortices in the stock, inserted the ballast, and epoxied it in. There wasn’t a lot of room for all that lead, but it did fit in the underside of the stock, and will be covered by the handle under-plate.

Once the epoxy has cured, I will reassemble the gun and slap on (whoops, I meant “carefully apply”) a finishing coat of varnish. Then its back to the water to:
• shoot targets,
• see how well the inhibitors stand up
• see how well the spear flies,
• feel how much thump there is on the wrist and,
• gather other general impressions.

Pictures soon.

Thanks for comments. Radiata pine (Pinus radiata) is not, as I understand it, the same as Norfolk Island pine. Its plantation grown around here and used mainly for house frames. Its not the strongest stuff in the world, but adequate to the task if the cross section is 40 millimetres and the grain is clear of checks and knots. You have to sort through a big stack to get the clear mouldings. I have used it in the past and it has stood up well.

The comments about spear whip and the use of one inhibitor only are food for thought. Especially since the Riffe spears I use have rest tabs. I plan changes/experiments with the number two incarnation, and this might be one. I can’t change too much though or else I can’t compare.


Ric
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Ric Fallu started spearfishing on the southern coast of Australia in the 1960s, and never really stopped. His other passion is building wooden spearguns.

Last edited by Ric.Fallu; 05-08-2012 at 01:52 AM. Reason: duplicate text
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