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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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Projectile specific gravity

Unread postAuthor: FighterAce » Sat Oct 16, 2010 8:59 am

I recently started casting plaster bullets. When I fire them, they almost fly sideways because I have a wrong twist rate on my rifling.
I found this calculator... http://kwk.us/twist.html that calculates the twist but I have no idea what SG I need to type in.

Is there any way to calculate SG? As far as I remember its got something to do with water...

And heres a pic of the bullet..

Image

Used a RTV silicone mold to cast.... the black ring is electrical tape... gives it a better seal.
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Unread postAuthor: POLAND_SPUD » Sat Oct 16, 2010 9:10 am

uhmm I think that SG is density
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 16, 2010 9:30 am

POLAND_SPUD wrote:uhmm I think that SG is density


Yep, specific gravity.

In order to calculate it, you need to divide the mass of its projectile by its volume (weight in grams, volume in mL)

To get the volume "using water", it's rather self explanatory:

Image
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Unread postAuthor: FighterAce » Sat Oct 16, 2010 9:50 am

Thanks!

So SG is density of projectile divided by density of water- witch is 1...
And density of projectile equals to mass divided by volume.

Sounds too simple to be true :D
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Unread postAuthor: Technician1002 » Sat Oct 16, 2010 10:55 am

If the object floats, it will displace it's mass in water, not it's volume. If your object floats, it will need submerged to get a volume displacement.

The electrical tape and projectile shape may be shifting the center of gravity too far to the rear. You may need nose weight or tail feathers to prevent the tumble. The shape may be unstable in flight.
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Unread postAuthor: POLAND_SPUD » Sat Oct 16, 2010 11:22 am

I recently started casting plaster bullets
The electrical tape and projectile shape may be shifting the centre of gravity too far to the rear
twist rate on my rifling


If I were you I'd switch to lead and drag stabilized projectiles... you don't even have to cast them - you can swage them



but if you really insist on using riffled barrel then change the design of the projectile. Switch to lead, make it shorter, change the shape (this one is more suitable for a hypersonic projectile... not something moving slower than the SOS)

Basically speaking it should look like these:
Image
:-D
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 16, 2010 11:39 am

POLAND_SPUD wrote:change the shape (this one is more suitable for a hypersonic projectile... not something moving slower than the SOS)


*cringe*

Did we have this epic debate for nothing?
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Unread postAuthor: Technician1002 » Sat Oct 16, 2010 11:42 am

Since the material is known, many items SG can be derived from it's composition.

Start here;
http://www.engineeringtoolbox.com/specific-gravity-solids-metals-d_293.html

Spoiler :roll: SG=1.8
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Unread postAuthor: POLAND_SPUD » Sat Oct 16, 2010 11:50 am

At typical spudgun velocities of say 300 - 700 FPS, the GL nose (a hemisphere) is as good as or better than the three pointier tips

sure... G7 is the best choice as far as cd is concerned but because it is longer it is more difficult to stabilise it


Clay min. SG 1.8
Lead SG 11.35

So by switching to lead you can increase the mass by a factor of ~6.3
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Last edited by POLAND_SPUD on Sat Oct 16, 2010 11:55 am, edited 1 time in total.
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Unread postAuthor: FighterAce » Sat Oct 16, 2010 11:53 am

It doesn't float... far from it

Total length is 77mm... center of gravity is 33mm from rear to front. The difference the electrical makes on the CG is negligible. Where should the CG be? Its not a dart and it will spin so it doesn't need to be all the way forward. I can always turn another projectile to make a mold from...

I already use drag stabilized projectiles...
And I know this shape was made for supersonic flight... I wanted it to be as efficient as possibile.

Lead is not an option for now... no resources to get tools, materials... and I don't use metal barrels.

Hypersonic is 5-6 times and more the speed of sound. The only projectile I know that reached hypersonic speeds was shot out of a rail gun :D

Edit. Damn you guys are really quick... so what shape was the winner in the "epic debate"? If there even was one...
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Last edited by FighterAce on Sat Oct 16, 2010 12:21 pm, edited 2 times in total.
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Unread postAuthor: POLAND_SPUD » Sat Oct 16, 2010 12:11 pm

Hypersonic is 5-6 times
yeah, sorry I meant moving faster than the speed of sound (that is Supersonic)

Anyway... if you don't use metal barrels then you can't use higher pressures. And if you can't use higher pressures... you get the idea

Since your projectile is light and relatively long it will be difficult to stabilize it in flight

Total length is 7.7mm
and what about the calibre ?

EDIT
uhmm the total length is 77mm, not 7.7mm, right ?
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Unread postAuthor: FighterAce » Sat Oct 16, 2010 12:20 pm

The pvc barrel I use is rated at 20 bar... I even used it at 30 bar.
Caliber is 1.27 mm and the weight is 7 gram.

uhh what a stupid mistake... 7.7mm hahaha
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Unread postAuthor: jackssmirkingrevenge » Sat Oct 16, 2010 12:26 pm

FighterAce wrote:what shape was the winner in the "epic debate"? If there even was one...


Image

Do you see any round-nosed bullets in this lineup of modern subsonic cartridges?

Anthony Williams wrote:Even this was not the ultimate answer as the bullet was still round-nosed and therefore quickly lost velocity and effectiveness at long range. Enter the current "Whisper" range of cartridges, designed to obtain the best ballistic performance possible within the constraint of subsonic velocity. The first to come to notice was the .30" Whisper (now known as the .300 Whisper), a .221" Remington Fireball cartridge (a short version of the standard 5.56 mm NATO case) necked out to 7.62 mm calibre to take a 210 grain boat-tailed spitzer match bullet.
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Unread postAuthor: POLAND_SPUD » Sat Oct 16, 2010 12:28 pm

then the twist rate should be about 1 in 3" at 650fps

so it seems I was right

@jsr
Do you see any round-nosed bullets in this lineup of modern subsonic cartridges?

do you see any plaster bullets? :)

If his bullet was made out of lead the ideal twist rate (according to that calculator) would be 1 in 7.5" which seems more or less reasonable

I suspect that his plaster projectile will disintegrate as soon as it leaves the barrel as it would have to spin at the rate of ~257 RPS (15 420 RPM)
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Unread postAuthor: FighterAce » Sat Oct 16, 2010 12:48 pm

I expected nothing more n nothing less JSR :D

According to GGDT I'm getting 222.2m/s or 728.35 fps so that would be 1 in 3.2"

POLAND_SPUD wrote:do you see any plaster bullets? :)

If his bullet was made out of lead the ideal twist rate (according to that calculator) would be 1 in 7.5" which seems more or less reasonable

I suspect that his plaster projectile will disintegrate as soon as it leaves the barrel as it would have to spin at the rate of ~257 RPS (15 420 RPM)


What else should I use? Concrete? Or even mix epoxy instead of water with plaster? :D

I can check that if I find a way to do it properly... my dremel goes to 35 000 rpm

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