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submersable speargun

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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submersable speargun

Unread postAuthor: homedepotpro » Thu Jul 19, 2007 3:24 pm

this gun has spare valve parts from previous guns but new pipe. it is a redesigned model of this:
Image
now it is safer and in my opinion, much cooler looking.
this is my current ammo
Image
i posted a topic here
http://www.spudfiles.com/forums/viewtopic,p,105256.html#105256
Specs:
-2'' chamber
-1/2'' barrel 27'' long
-1'' inch modded sprinkler valve
-it has a sort of coupling that fits over the muzzle and holds a thin plastic sheet, which keeps water out the the barrel before the spear is shot
pics:
Image
Image
Image
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Unread postAuthor: benstern » Thu Jul 19, 2007 3:26 pm

There might be significant accuracy decrease when the ammo exits the barrel due to the crazy physics of air/water interfaces.

Tthats why normal spear guns use springs.
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Unread postAuthor: mopherman » Thu Jul 19, 2007 3:27 pm

wow, cool orange blowgun. 100 times better than the last one.
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Unread postAuthor: potatoflinger » Thu Jul 19, 2007 4:56 pm

That's cool, it's the first underwater gun I've ever seen.
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Unread postAuthor: paaiyan » Thu Jul 19, 2007 5:06 pm

Well I think most pneumatics could technically work underwater, if you think about it. i mean the concept of a pneumatic gun requires that it be airtight. I will say, however, this is the first gun I've seen made specifically to be used underwater. And since you're using it in water, your projectile will actually be called a harpoon.

EDIT: If you are going to be holding it underwater, you might want to let the barrel fill with water, then load the projectile. Then the effect of the air/water interface should be negligible
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Unread postAuthor: homedepotpro » Thu Jul 19, 2007 5:10 pm

yes but then it increases the mass of my projectile by about 100 times since i have to shoot all the water out too
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Unread postAuthor: paaiyan » Thu Jul 19, 2007 5:19 pm

Well it wouldn't be by 100, but it would be considerably heavier. If you don't want to do that, then I would cover the end in cellophane or something, because if the barrel fills partially with water, then it'll really throw it off. And since you're using that harpoon underwater, I would suggest adding something at the back there to bevel the plastic cap. If that's flat, it'll drag a lot, but if you were to put something on it, say a cone made of soda bottle plastic or something, your harpoon would be much more aeordynamic, er, water...o...dynamic? It would swim better...
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Unread postAuthor: Gepard » Thu Jul 19, 2007 6:03 pm

Make the dart hollow and then sleeve it on the barrel.

That way you can fire it without messing about with plastic and don't have the problem of it suddenly hitting water.

Michael
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Unread postAuthor: homedepotpro » Thu Jul 19, 2007 6:03 pm

yeah i do have a cover for the barrel, i mentioned it in my initial post, i am also just about to put cones on the darts, ha, great minds think alike. btw, i think its hydrodynamics.

they i would have to completely redesign the gun and ammo
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Last edited by homedepotpro on Thu Jul 19, 2007 6:17 pm, edited 1 time in total.
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Unread postAuthor: Gepard » Thu Jul 19, 2007 6:07 pm

No make the spear the cover.

Hydrodynamics isn't to do with an projectile going from a less dense medium to a more dense medium.

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Unread postAuthor: homedepotpro » Thu Jul 19, 2007 6:18 pm

no the force of water are hydrodynamics, while the forces of air are aerodynamics.
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Unread postAuthor: Gepard » Thu Jul 19, 2007 6:28 pm

No....hydrodynamics is the study of a moving liquid as opposed to hydrostatics - where the liquid is stationary. We're talking about the change from a less dense to a more dense medium by the spear and as I said that isn't hydrodynamics.

You wouldn't need to redesign the gun just the ammo.....

Michael
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Unread postAuthor: iknowmy3tables » Thu Jul 19, 2007 6:31 pm

you can use a condom to cover the barrel :lol: but you really can there are condoms for guns for under water purposes
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Unread postAuthor: homedepotpro » Thu Jul 19, 2007 6:37 pm

what i have is work and it cheap, thats 3tables. i have heard of those before though
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Unread postAuthor: Big-mac » Thu Jul 19, 2007 6:47 pm

why not use sharpened 1/2 inch wodden or even 1/2 steel rod. it would seal your barrel and dont forget to grease it for less friction.
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