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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: al-xg » Sat Oct 16, 2010 12:58 pm

Length and low density are the culprits here.
I've shot quite a few bullets out of my rifled copper barrel, with pretty good results most of the time. I mostly used lead (or lead/tin alloy), but did also try much lighter materials like paraffin wax.
I was able to test the lead bullets past 80m and they were still stable (even after exiting two planks of pine used as targets.), but the wax bullets sometimes didn't even make it to the target.

Although the longer projectile should produce less drag, the extra weight reduces muzzle velocity, so I found shorter bullets more effective (countering bullet drop does make it more difficult to shoot accurately at different ranges)
Also the longer bullets need faster rotation to stabilise and with lowish velocities that means crazy twist rates.
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Unread postAuthor: POLAND_SPUD » Sat Oct 16, 2010 12:58 pm

well if think you can get it to be stable in flight then go for it
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Unread postAuthor: Ragnarok » Sat Oct 16, 2010 1:03 pm

Poland, you seem to have goofed on the maths, presumably by calculating as if it were one turn in 3 FEET, not 3 inches.

If you do it right, that's ~160,000 RPM, and I cannot see it remaining intact at those spin speeds.
As centripetal force has to increase with the square of angular velocity, the projectile needs to be 20 times stronger to survive 160,000 RPM than it does 35,000 RPM.

I just can't see that it's going to survive those spin rates. (If it's not outright destroyed by skipping over such steep rifling).
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 16, 2010 1:11 pm

FighterAce wrote:Just did a little test... the plaster binded with water and glue takes 35 000 rpm No Problem.


Have you taken into account the heat and stress generated with friction with the barrel?

And I agree with Rag, such a large length:width ratio needs an extremely high twist rate to stabilise it.

Take theseextra long 22LR rounds, they need almost double the twist rate of normal 22 rifles to stabilise adequately.
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Unread postAuthor: FighterAce » Sat Oct 16, 2010 1:11 pm

al-xg wrote:Although the longer projectile should produce less drag, the extra weight reduces muzzle velocity, so I found shorter bullets more effective (countering bullet drop does make it more difficult to shoot accurately at different ranges)


Lighter bullets go faster but lack the range... heavier bullets go slower but can reach out and touch you.

So 1 in 3.2" rate it is... I'm planning on using an all thread rod with a hex nut machined at a certain angle to make grooves.
Any formulas to calculate what this specific angle should be to achieve 1 in 3.2 twist rate?
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Unread postAuthor: POLAND_SPUD » Sat Oct 16, 2010 1:11 pm

ohh damn... that's why I hate imperial units

And I agree with Rag

then you also agree with me as I suggested it from the very beginning :wink:
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 16, 2010 1:18 pm

POLAND_SPUD wrote:then you also agree with me as I suggested it from the very beginning :wink:


On this point, yes - I don't disagree with you for the sake of it y'know ;)
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Unread postAuthor: FighterAce » Sat Oct 16, 2010 1:26 pm

Ahh.. ok.. so I need to make it shorter..
Any ideas how much RPM plaster could take? If it were 1 inch in length then it would get ~63 000 RPM... still too much huh :?
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Unread postAuthor: Ragnarok » Sat Oct 16, 2010 1:39 pm

FighterAce wrote:Any ideas how much RPM plaster could take?

No idea. I'd need to know the tensile strength of plaster to work that one out, and that's not the kind of figure I have on hand.
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Unread postAuthor: FighterAce » Sat Oct 16, 2010 1:43 pm

Ragnarok wrote:
FighterAce wrote:Any ideas how much RPM plaster could take?

No idea. I'd need to know the tensile strength of plaster to work that one out, and that's not the kind of figure I have on hand.


What about concrete?
I mentioned it in a joke but now I'm starting to think more about it...
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Unread postAuthor: Ragnarok » Sat Oct 16, 2010 1:49 pm

FighterAce wrote:What about concrete?

Concrete isn't good in tension - it normally needs to be reinforced with steel stressing wires (which take the tension instead) to make it any good for that kind of thing.
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Unread postAuthor: FighterAce » Sat Oct 16, 2010 1:57 pm

But isnt it a better choice then plaster?

What else could you recommend? My only requirement is that its not melted.. like lead...
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Unread postAuthor: Ragnarok » Sat Oct 16, 2010 2:03 pm

FighterAce wrote:But isnt it a better choice then plaster?

Yes. But whether it's good enough... I'd have to work that one out.
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Unread postAuthor: POLAND_SPUD » Sat Oct 16, 2010 2:26 pm

My only requirement is that its not melted.. like lead...

swaging bullets
http://www.svartkrutt.net/articles/vis.php?id=32

I mentioned it on the first page
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Unread postAuthor: Fnord » Sat Oct 16, 2010 2:31 pm

Bondo would be good. Not super tough, but it doesn't shatter very easy. If you want something heavier just cast bondo with a metal core(this will raise the rpm speed required for stability, but not to an unreasoable amount).
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