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[Tutorial] Screw Darts

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Unread postAuthor: jackssmirkingrevenge » Thu Jun 13, 2013 1:26 pm

wyz2285 wrote:So it is possible to make the helsing :D


Hehe believe it or not I was thinking that recently!

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I could make 5 of these, and put them on a revolving chassis...

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Unread postAuthor: Alanstone » Thu Jun 13, 2013 1:41 pm

While we are on the topic of a projectile center of gravity, I have a design consideration. A hollow projectile driven by a pusher ( plus you can play add infinitum with the projectile dynamic) . Low mass, little drag vs large barrel diameter. Is there any merit to this? I was sure I had found the holy grail half way into a bottle of rum. The next day I wasn't so sure.
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 13, 2013 1:49 pm

Tubular bullets are an existing concept, like for example the PMC ultramag:

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"cookie cutters" :D

Apparently they could go through body armour with ease, not surprising given the high sectional density and small frontal area. You'd need rifling for them to fly straight though...
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Unread postAuthor: wyz2285 » Thu Jun 13, 2013 2:58 pm

Hehe believe it or not I was thinking that recently!

Believe or not I was building it recently, but as I was fitting the arrows over a smaller diameter tube, barrel strength was a problem. But now I can confidently use conventional projectile inside barrel configuration. (actually I asked if I could and somebody have said no :roll:)
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CpTn_lAw wrote::D "yay, me wanna make big multishot pnoob with 1000 psi foot pump compressor using diamond as main material. Do you think wet bread make good sealant? " :D
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Unread postAuthor: Alanstone » Thu Jun 13, 2013 3:08 pm

You'd need rifling for them to fly straight though...

That must depend on design surely?
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 13, 2013 4:24 pm

Alanstone wrote:That must depend on design surely?


Having air flow through the projectile would preclude drag stabilisation, you'd have to put fins on it.

That's the theory but since I believe in science by doing, whipped up a quick test projectile with a heavier brass nose and aluminium tail, hollowed out.

The cross marks the centre of gravity, as a solid projectile it should have flown straight. As you can see though, fired at a cardboard box just 5 metres away, it was clearly tumbling.
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Unread postAuthor: dewey-1 » Thu Jun 13, 2013 5:32 pm

JSR

Plug the brass end with Delrin nose piece to verify that being hollow is actually the problem!
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Unread postAuthor: jackssmirkingrevenge » Thu Jun 13, 2013 11:22 pm

dewey-1 wrote:Plug the brass end with Delrin nose piece to verify that being hollow is actually the problem!


Right you are, I forgot control group A!

I didn't do it because of previous experience with similar projectiles but yes, for completeness' sake it should be done, eventually when I get home.
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Unread postAuthor: Alanstone » Thu Jun 13, 2013 11:44 pm

Thank for the links to the ammo JSR. I didn't know it existed. As to my proposed idea (the drawing I did) I thought about the projectile needing fins especially because its short with a large diameter. That's why, in the drawing you see material inside at the back of the projectile in my drawing. I thought that perhaps that would create the drag needed to stabilize it. Especially if it was designed to induce spin. Do you think that if the projectile had an airofoil shape it would help stabilize the projectile? That and fins inside the back of the projectile. It would probably have to be the other way round though ( with the inside fins at the front of the projectile to put the cg forward rather than create drag. I don't know. :( My mind keeps showing me an airofoil shape, like a stretched teardrop as a profile shape. I wish I had a 3d printer. What do you think Dewey? Would an airofoil profile help stabilize the profile? Could fins inside the projectile make it spin?
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 14, 2013 12:39 am

I think this thread is right up your street: http://www.spudfiles.com/forums/long-ra ... 24104.html
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Unread postAuthor: Alanstone » Fri Jun 14, 2013 1:10 am

Thanks for the link JSR. I am starting to get used to the idea that all my "new and innovative" thoughts are old hat! I lolled at myself this morning. :)
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 14, 2013 1:45 am

Alanstone wrote:Thanks for the link JSR. I am starting to get used to the idea that all my "new and innovative" thoughts are old hat!


That's why it's good to research - you have an unbelievably extensive world of knowledge unprecedented in human history at your fingertips, use it ;)

It amazes me how businesses give their employees internet access and expect them to work :roll:
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Unread postAuthor: Alanstone » Fri Jun 14, 2013 4:55 am

Fair enough. In my defense I must say I naively believed I had stumbled onto something unique.
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Unread postAuthor: jackssmirkingrevenge » Fri Jun 14, 2013 5:03 am

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Unread postAuthor: jackssmirkingrevenge » Fri Jun 14, 2013 10:21 am

Further to the "cookie cutter" test, as dewey suggested I plugged the nose of the projectile with some foam to give practically identical CG but a closed tube shape and shot it at the same target, two sides of a carboard box. As you can see it was clearly travelling nose first.
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