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Ideal Length for Cylindrical Ammo

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Ideal Length for Cylindrical Ammo

Unread postAuthor: goose_man » Wed Oct 10, 2007 6:23 am

The title basically explains my question, but here it is anyway:
If I shoot a cylindrical homogeneous projectile out of a smoothbore barrel, what length is ideal for a straight flight?

I'm thinking that the ideal length would be near the diameter of the projectile, but maybe not exactly it.

Can anyone help me figure out the exact diameter-to-length ratio?

-goose_man
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Unread postAuthor: Hubb » Wed Oct 10, 2007 6:27 am

For a straight flight, you will probably want a projectile that is longer. It would seem to me that the longer the projectile, the straighter it would fly. Of course, the ammo could also be modified to fly straight, like adding a tail.

As far as the exact diameter-to-length ratio, I have no idea.
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 10, 2007 6:39 am

It would seem to me that the longer the projectile, the straighter it would fly.


Nope - A cylindrical homogenous projectile will start to tip over the instant it leaves the barrel, irrespective of how long it's been been moving straight in the barrel.

In order to fly straight without fins, you need the centre of gravity to be approximately 25% of the total length away from the nose. You can acieve this by either hollowing out the rear section or using a heavier material for the nose.

Can anyone help me figure out the exact diameter-to-length ratio?


Rocketry folk seem to favour a 1:14 ratio for optimum performance.
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Last edited by jackssmirkingrevenge on Wed Oct 10, 2007 8:21 am, edited 1 time in total.
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Unread postAuthor: Hotwired » Wed Oct 10, 2007 8:03 am

jackssmirkingrevenge wrote:
It would seem to me that the longer the projectile, the straighter it would fly.


Nope - A cylindrical homogenous projectile will start to tip over the instant it leaves the barrel, irrespective of how long it's been been moving straight in the barrel.


My polymorph slugs don't like doing what they're meant to then :P

Perfect circles cut in cardboard sheets from 30m away and they're ~14.5mm dia and 20mm long square faced cylinders.
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 10, 2007 8:28 am

I said it will start to tip over the instant it leaves the muzzle, not start doing backflips immediately :P

I remember seeing a bit of his speed footage in this documentary where a cylindrical slug was fired out of a smoothbore barrel, it travelled a good few metres before the rotation forward was noticeable.

It's possibly that the relatively short length of your slugs in relation to their diameter gives less leverage for the air to push on and allows them to rotate at a slower rate. Have you tried at 60 metres?
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Unread postAuthor: jimmy101 » Wed Oct 10, 2007 12:33 pm

Hmmm, wouldn't a long thin cylinder take a bit longer to start tumbling than a shorter one?

If the total mass is the same, the long thin shell has a larger moment of inertia than the shorter one. It takes more energy to spin a long thin cylinder than a shorter/fatter one doesn't it?

Eventually though, all cylinders (of constant density) start to tumble. I would thing that how long it takes to start tumbling and how fast it tumbles (on average) would depend on a bunch of things. Length to width ratio, surface roughness, velocity, presence of any spin or tumble tendency caused by imperfections in the barrel etc.

I've shot 3/4" copper tubing with end caps (~3" total length) from my compressed air "snipper". Without nose weight the ammo punches a rectanglular hole in a piece of plywood at a range of only a couple feet. In other words, the round tumbled ~90 degrees in a few feet.
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 10, 2007 1:44 pm

This sounds like an ideal subject for a school project investigation :D
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