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Projectile specific gravity

Post about things you have launched or thought about launching. Also post about various materials used for building cannons. No posts about explosive projectiles!
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Unread postAuthor: Ragnarok » Sat Oct 16, 2010 2:40 pm

Fnord wrote:this will raise the rpm speed required for stability, but not to an unreasonable amount.

Other way around. More dense projectiles need slower spin rates.
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Unread postAuthor: Fnord » Sat Oct 16, 2010 2:49 pm

Ragnarok wrote:Other way around. More dense projectiles need slower spin rates.


My bad. I should have said "higher rpms than a projectile of the same density".

With a metal core most of the mass will be toward the center; you'll have less of a gyroscopic effect because of this.
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Unread postAuthor: Technician1002 » Sat Oct 16, 2010 5:26 pm

I do know the RPM is quite high as evidenced by this video.

[youtube]http://www.youtube.com/watch?v=foZlciP6gUQ[/youtube]
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Unread postAuthor: FighterAce » Sun Oct 17, 2010 6:04 am

What do you think about this? Length is 30mm, caliber is still 12.7mm.

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Looks like a little coconut...
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Unread postAuthor: al-xg » Sun Oct 17, 2010 6:25 am

I tend to make shorter bullets a little less pointy in order to have at least one calibre's length at full calibre... :?
Or even If the whole surface isn't full bore at least two contact lines a calibre a part to make sure the bullet and barrel axes are in line.


That shooting ice thing is pretty amazing.
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Unread postAuthor: FighterAce » Sun Oct 17, 2010 6:36 am

This is not by any means finished. Just throwing ideas out there and asking for some feedback.

What is your ratio of caliber to projectile length?
I dont have much room on here to use electrical tape... Do you think O rings would do as contact lines?
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Unread postAuthor: Crna Legija » Sun Oct 17, 2010 7:12 am

you could use epoxy most of the companys list there strengths on the web site.
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Unread postAuthor: FighterAce » Sun Oct 17, 2010 9:05 am

Something like this... ?

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At least the O rings are cheaper than epoxy :D
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Unread postAuthor: al-xg » Sun Oct 17, 2010 9:19 am

That looks better, but I'm guessing it's going to be a bit of a pain to make any number of these bullets ?

Do you have a soft target to shoot at ? I'd be interested to see how that projectile does from a PVC barrel.

A denser material would be better, but can't think of anything right now that doesn't require casting with highish temperatures...
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Unread postAuthor: FighterAce » Sun Oct 17, 2010 9:24 am

I will make a mold from a similar bullet and then cast it in cement... if I can get a fine enough sand... and embed a nail in the center.

Another problem is the rifling... I have no idea what angle to put on the cutting surface to achieve a desired twist rate.
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Unread postAuthor: al-xg » Sun Oct 17, 2010 9:48 am

Picture a cross section of the barrel. Over half a turn the rifling goes from one wall of the barrel to the other forming a right angled triangle, the diameter and half turn distance are known so trigonometry can be used to work out the angle.

So Atan(d/(twist*25.4/2)*0.017453=rifling angle in degrees, assuming d in mm, twist in inches and calculator(or Excel) set to radians.
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Unread postAuthor: FighterAce » Sun Oct 17, 2010 9:59 am

al-xg wrote:trigonometry


We didnt get that far in school so I have no idea how to calculate it...

al-xg wrote:So Atan(d/(twist*25.4/2)*0.017453=rifling angle in degrees, assuming d in mm, twist in inches and calculator(or Excel) set to radians.


:shock: Too much math

How can you calculate with mm and inches and expect a good result? Doesnt it all have to be in SI?
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Unread postAuthor: al-xg » Sun Oct 17, 2010 11:11 am

atan , arctan or tan-1 is just a button on the calculator, it will do the hard work.
The calculator often can be set to degree or radians.
To get the right value for the angle, degrees should be used, but if the calculator or program can't be changed from radians(ie in Excel) 0.017453 is used to convert it back to degrees.

The inches are being converted to mm by multiplying them by 25.4 (1inch=25.4mm). The formula doesn't have to be in a particular standard as it involves a ratio, as long as diameter and twist are made to have the same units it should work.

Note the twist is also divided by 2 as it is the half turn length we need.

Trying to simplify to the most common situation:

Atan(d/(twist/2))= rifling angle (with d in mm, twist in mm and calculator set to degrees gives an angle in degrees)
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Unread postAuthor: Ragnarok » Sun Oct 17, 2010 12:01 pm

al-xg wrote:Atan(d/(twist/2))= rifling angle (with d in mm, twist in mm and calculator set to degrees gives an angle in degrees)

Afraid not. As this "triangle" is around the circumference of the barrel, not across the diameter (rifling spirals, not zig-zags), it needs to include Pi in there.

My reckoning is that it needs to be atan(calibre*Pi/(twist)) - with both calibre and twist rate in the same units, and the calculator set for whichever units of angle you wanted it to give your answer in.
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Unread postAuthor: SpudFarm » Sun Oct 17, 2010 12:15 pm

What do you think about this? Length is 30mm, caliber is still 1.27mm.

Does not make sence..
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