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spudgun range, are we falling short?

A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 29, 2011 4:06 am

Thanks for all the input guys.

Again, I would love to make this an official contest and would be more than happy to put up a cash prize (and maybe another pen gun :roll:) but I can see things getting ugly in terms of validating claims.

IMO, total distance shouldn't be the sole objective in designing a projectile. A round should be accurate enough to strike a selected target and pack enough energy to do some "work" when it encounters that target.


In general I agree - in this case, the great thing about projectiles designed to achieve substantial range is that the are also ideal penetrators in terms of form and density.

How difficult is it to construct very low drag projectiles and what would a good drag coefficient be for shooting long distances?


If you can build a decent launcher, a matching projectile should not be an issue, especially if you don't have a requirement for consistency and mass production.

Image

Compare the sphere to a streamlined body, you can input the Image data for different shapes into GGDT's ballistic calculator and marvel at the difference it makes in range.

Some streamlined bodies that would make good projectiles, if you could make it stable by altering centre of gravity.

Image
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Unread postAuthor: ilovefire » Tue Mar 29, 2011 4:36 am

make a cone shape out of plastic sheet then fill with epoxy then drop a ball bearing in that has been sanded on one side in, once set remove plastic cone?
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why make it if it dosent shoot?
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Unread postAuthor: LeMaudit » Tue Mar 29, 2011 6:37 am

About bullet shape/distance.

I was hesitating between a round ball and a bullet shape like the Hornaby Great plains in .50 Cal.

My understanding is that the bullet shape is made for riffled barrel, and will tumble during flight without a spinning effect. But is this true for a "slow" speed, say 500 fps?

Or is there a gain in distance/precision using it instead is a simple lead round ball?
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Unread postAuthor: Crna Legija » Tue Mar 29, 2011 6:43 am

yea it will tumble at any speed but you can drill out the back to move the CG forward which will make it stable, it will make it lighter tho.
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 29, 2011 6:48 am

LeMaudit wrote:My understanding is that the bullet shape is made for riffled barrel, and will tumble during flight without a spinning effect. But is this true for a "slow" speed, say 500 fps?


Without fins or some other form of drag stabilisation, an unrifled projectile will start to tumble as soon as it leaves the muzzle regardless of velocity. It can be clearly seen in this high speed footage.

Or is there a gain in distance/precision using it instead is a simple lead round ball?


A tumbling cylindrical bullet has a higher drag coefficient than a sphere so unless it's stabilised, it's better to go with spheres. The disadvantage of spheres is that for a given material and barrel diameter, you're limited to a certain weight and therefore sectional density.
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Unread postAuthor: ilovefire » Tue Mar 29, 2011 6:50 am

i did something similar today i cut the back of of a bullet then melted out most of the lead and left some in the nose so that the copper shell of the bullet will hopefully act as a flight
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 29, 2011 7:02 am

ilovefire wrote:i did something similar today i cut the back of of a bullet then melted out most of the lead and left some in the nose so that the copper shell of the bullet will hopefully act as a flight


That should work, I remember MrC was shooting 0.303 bullets with the empty cartridge casings attached to have a forward CG and they seem to have worked well.

On the subject of drag reduction, some interesting work by hippies.
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Unread postAuthor: ilovefire » Tue Mar 29, 2011 7:09 am

ilovefire wrote:make a cone shape out of plastic sheet then fill with epoxy then drop a ball bearing in that has been sanded on one side in, once set remove plastic cone?


do you think my earlier suggestion would work well?
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 29, 2011 7:20 am

ilovefire wrote:do you think my earlier suggestion would work well?


Assuming the CG is forward of the CP, it should.

A simple way to calculate the CP:

The easiest way to find the center of pressure is to make a cardboard cutout of the model rocket, out of a thin, flat, stiff piece of cardboard. Using a ruler, or similar object, balance the cutout. The point at which the cutout balance will approximate the center of pressure of the rocket


For a "teardrop" shape, it's going to be pretty far forward, so I'm not sure if the difference in density between steel and epoxy will be enough to ensure stability.
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Unread postAuthor: LeMaudit » Tue Mar 29, 2011 7:22 am

So a simple drilling to a back of a lead bullet like that would both reduce weight (good, as I don't want it too heavy to keep some reasonable speed) and improve distance/accuracy?
I understand well that this would move the center of gravity to the tip, but this would not make any kind of fins. Would it not tumble too?

I was only wondering if the hop-up / Magnus effect is used for big-bore (I have in mind 50 cal at somehow 500 pfs). All I can find is about Airsoft/Paintball. Is it used in heavier bullets?
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Unread postAuthor: ilovefire » Tue Mar 29, 2011 7:25 am

hmm well maybe ill give it a go and see hoe it works
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Unread postAuthor: Technician1002 » Tue Mar 29, 2011 7:28 am

jackssmirkingrevenge wrote: A tumbling cylindrical bullet has a higher drag coefficient than a sphere so unless it's stabilised, it's better to go with spheres. The disadvantage of spheres is that for a given material and barrel diameter, you're limited to a certain weight and therefore sectional density.


I have found that cans of soda pop soon turn sideways and are reasonably stable in that position. All high lobs from the air cannon have belly flopped for a landing with no exceptions.

Falling pop cans always land on the side.
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Unread postAuthor: LeMaudit » Tue Mar 29, 2011 7:34 am

Technician1002 wrote:I have found that cans of soda pop soon turn sideways and are reasonably stable in that position. All high lobs from the air cannon have belly flopped for a landing with no exceptions.


Really? :shock:

Hey, that could mean a square/rectangular barrel firing cylinders sideways would work? How awkward... love it! :D
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Unread postAuthor: Crna Legija » Tue Mar 29, 2011 7:46 am

yea if you drilled that round out like in your edited pic it would be stable, think about air rifle pellets how they are a empty cone shape.
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Unread postAuthor: jackssmirkingrevenge » Tue Mar 29, 2011 7:46 am

LeMaudit wrote:So a simple drilling to a back of a lead bullet like that would both reduce weight (good, as I don't want it too heavy to keep some reasonable speed) and improve distance/accuracy?


Yes - this is how most shotgun slugs work, and give good results from smoothbore barrels.

Image

I understand well that this would move the center of gravity to the tip, but this would not make any kind of fins. Would it not tumble too?


It would try to tumble, but since the centre of pressure is behind the centre of gravity, the air will push it back along the correct axis of travel as in the attached diagram.

You can see this effect in some shots of USG's compound rounds where the round is visibly fishtailing.

I was only wondering if the hop-up / Magnus effect is used for big-bore (I have in mind 50 cal at somehow 500 pfs). All I can find is about Airsoft/Paintball. Is it used in heavier bullets?


I've never see it being used for anything other than airsoft projectiles.
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