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Lathe turned pellets

A place to ask general spud cannon related questions.
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Unread postAuthor: Ragnarok » Mon Jun 08, 2009 2:24 pm

john bunsenburner wrote:Would you recommend working more on the projectile design or the gun design, which one of the two is more important, easier?

To get a projectile out to that kind of range with a moderately flat trajectory, power is essential. A good projectile design will allow you to get away with less of it however.

For the 400 metre ranges I'm looking at, I'm using velocities of around 270 m/s (~900 fps), and a dart with a ballistic coefficient of ~5000 kg/m<sup>2</sup>. For comparison, a really good .308" bullet might be 2000 kg/m<sup>2</sup> (Bear in mind, unlike drag coefficient, higher is better with ballistic coefficient).
To put that into a slightly more useful figure, the dart's BC means it'll travel the 400 metres with ~95% velocity retention.

In short, those ranges take very respectable velocity, towards the practical limits of what pneumatics can manage, even with BCs as good as we're discussing. Even in this case, I think my set up is reasonably marginal as far as actually getting the projectile out there with a flat trajectory. It'll still take a ~1.6 degree launch angle, which is pretty big as far as this kind of thing is concerned - outside of the adjustment range of most scopes, so it'll need some form of shimming.

Now, if you want to use commercial pellets, there's a problem there. Air rifle pellets are about 100 kg/m<sup>2</sup> in domed .177" and 150 kg/m<sup>2</sup> in domed .22", which seriously puts restrictions on the potential range.
A good commercial air rifle might put either calibre to 900-1000 fps while still keeping decent accuracy, but even so, reasonably accurate ranges are limited to within 200 metres. And that of course assumes you can make those velocities in the first place.

Really, you want to be making at least 800 fps, and you'll need a pretty impressive ballistic coefficient to go with that. That is to say, both things are important.
Bear in mind though, high ballistic coefficient projectiles can still take quite a lot of power to accelerate, because they've got to have some mass to make up a respectable sectional density.
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Unread postAuthor: john bunsenburner » Mon Jun 08, 2009 2:38 pm

Well GGDT predicts nearly arond 250m/s, im hoping this will be slightly better in the real gun seeing as how many parts I can't really model too well. So the "power" shouldn't bee too much or a problem. Now what would i have to keep in mind to get a projectile that has a high(er) BC than pellets?
How are you making your darts, or are they purchases? If so where?
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Unread postAuthor: Ragnarok » Mon Jun 08, 2009 4:42 pm

john bunsenburner wrote:So the "power" shouldn't bee too much or a problem.

That entirely depends on the mass of the projectile you're modelling. If the projectile isn't heavy enough to have a good sectional density, then the velocity is of little use.

In the end, there is going to be an ideal balance between velocity and projectile mass for any given range. Working it out precisely is a bit more tricky however.

Now what would i have to keep in mind to get a projectile that has a high(er) BC than pellets?

Greater sectional density and lower drag coefficient.

Sectional density is proportional to mass and inversely proportional to cross sectional area. A heavier or narrower projectile therefore has a better BC.
However, if you don't want saboted projectiles, then you'll just have to increase mass.

Drag coefficient isn't too complex to improve, within reason. Avoiding sharp edges, angling faces away from the air flow, and "boat-tails" are the main steps to take.

How are you making your darts, or are they purchases?

They're entirely custom. I've been designing them on and off for years now.

But that time does show, with a BC of 5000+ kg/m<sup>2</sup>, nigh immunity to wind, high stability, potentially sub-MOA accuracy, decent resistance to minor flaws in construction and the capacity to pierce 6+ mm of steel plate.

Their actual construction is slightly up in the air at the moment. I might be pulling in a few favours to get the various parts of them machined or laser cut as appropriate.
Then again, I'm tempted to NOT go that way and just prove that it can all be done with fairly standard tools.
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Unread postAuthor: jaythedogg » Mon Jun 08, 2009 11:13 pm

If I were to machine a pellet it would look like this:

Image

What you would do is machine the pellet from steel.

Machine a hollow block with one end open, out of steel. A half an inch longer than the pellet & half an inch wider than the pellet on each side. The open end needs to be completely open, so the inside walls are smooth with the edges of the opening.

Cut it in half.

Drill a small hole in the very tip of your pellet & in the exact center of your mold block's closed end opposite of the open end. Same drill bit.

Drop a steel pin that matches the hole drilled into the mold block hole, then mount your pellet on it (so it is sitting on it's nose, skirt/open end facing you out of the open end).

There is your mold skeleton.

Melt down some scrap aluminum (check instructables.com---Very easy) & making sure that the mold is firmly clamped together & COMPLETELY dry (hot metal explosions SUCK but are easily avoided with proper precautions), then fill it to the top of the pellet skirt, but don't overflow into the pellet's hollow end.

Let it cool for 30 minutes. Remove your clamps from the mold skeleton, insert a screw driver if needed & gently tap it in until you crack the skeleton from the molded block.

Take the aluminum block you made (CAUTION, the block may still be hot!) & run it through a fine metal saw, right down the middle, top to bottom. You want to remove as little of the aluminum as possible during sawing, so use the finest aluminum grade band saw blade as possible, at high speed with a guide so it stays straight & true.

Next, make yourself a set of handles from an old pair of slip joint pliers, which requires you to machine a slot in both sides of the mold block & removing some metal from the jaws of the pliers so when closed it holds the mold block halves perfectly together... The tap & drill the mold block & affix your pliers with a set screw or two.

Once finished, you can (go outside) smelt some dry lead (again, melting metals plus water equals bad things) & make your own lead slugs, as an aluminum mold block will withstand molten lead easily.

Every 10 seconds to a few minutes (depending on the thickness of the slugs) you can pop the slug out, give the aluminum (in the open position) a 30 second break (or less with a fan) to cool & cast another.

Once your slugs are complete, put them into a hollowed wood holder in your bench drill press' vice & use a properly sized drill bit to hollow out the pellet, which provides more push with less tail weight.

Just remember when shooting lead slugs out of a barrel (or any slug for the matter) it never hurts to grease the outer edges with machine grease (Moly grease actually is the best). Less friction=less drag=more velocity=higher energy transfer to the target=more destruction pics for us!

Hope this wasn't too strung out, & it sounds more complicated than it really is. The toughest part is machining the steel, but any steel, heck even brass would work for the skeletal mold, so long as it melts a couple hundred degrees higher than aluminum (which is virtually anything minus copper, it's close.)

Hope this helps!

-James

P.S. Yes, skirts provide rear stability & the domed point provides less wind resistance while providing a smooth domed top for the wind to slide over, giving more stability.
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Unread postAuthor: john bunsenburner » Tue Jun 09, 2009 1:00 pm

hmm, im having a bit of an idea here:

if i made a dart, fully out of lead that has finns that instead or strait are angled in one way so that the blow-by would make the dart spin extremly fast providing stability while there will be no sharp edges ect. I am not sure how will this would work with longer distances, but for shor distance this should be very accurate. Remembre I am using only a 5mm bore, GGDT predicted the 250m/s for a projectile that has a weight of 0.7g, the weight of a pellet according to the internet. Really I assume the oly way to find out is to try.
What do you think rag?

At jay: Thanks for that, I know it would work well however making a molds for pellets is not my problem it is making a projectile that fulfills my needs, THEN you can help me with the mold. If I am goign to use pellets I might as well buy them. Also I have a waste vegtable oil plan that i'v been planning ot bilt for a while, so melting metal is not a problem. Last but nto least, I am sure that molten alli wouldn't really cause copper to deform too much after all it does have a fairly high melting point compared to alli.
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Unread postAuthor: jeepkahn » Tue Jun 09, 2009 1:29 pm

Spin induced by fins hurts velocity, it takes energy to impart spin... Make you're darts from tungstun rod, and then you could braze a tail cone onto it, and you'd have a weiner... infact it could just be brzing filler that makes the tail "wedge"... with a high fps, if the tail diameter is .22 then the main shaft coud be .20 and it would fly straight...
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Unread postAuthor: john bunsenburner » Tue Jun 09, 2009 1:33 pm

Tungsten rod sounds expensive as hell, and hard to machine... I know it would hurt velocity but it is sure to help accuracy is it not?
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Unread postAuthor: jeepkahn » Tue Jun 09, 2009 1:52 pm

actually, in the grand scheme of things, go get you some 6d finishing nails, chuck them in the lathe and turn the heads down to whatever calibre you need them, and you'll be surrprised how accurate they are....
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Unread postAuthor: Ragnarok » Tue Jun 09, 2009 6:31 pm

john bunsenburner wrote:If I made a dart, fully out of lead that has fins that instead or straight are angled in one way so that the blow-by would make the dart spin extremely fast providing stability while there will be no sharp edges etc. I am not sure how will this would work with longer distances, but for short distance this should be very accurate.

That's one of the core principles of my projectile design - although I'm not using lead.
It's a simple idea with a lot of promise, but you need to think into it quite deeply.

Firstly, to correct jeepkahn's misconception, done right, it can actually REDUCE drag relative to other fin designs. I've got drag off the fins down around 90% relative to a standard design.
It's still not something for nothing however. It's just much more efficient and puts most the energy it still uses to a practical use. That is, it's more drag than no fins, but less than other designs.

Now to move on, darts are not something for which spin stabilisation is a good idea, because long, thin projectiles need very high spin rates to remain stable.
The stability should still come from drag stabilisation, and the spin should only be used to compensate for aerodynamic flaws in the projectile.

This means you have to balance the gyroscopic spin of the projectile against the drag on the fins. That is, you need to try and avoid spin rates too high, or ones which are naturally unstable to avoid causing issues with the stability of the projectile.

Remember I am using only a 5mm bore, GGDT predicted the 250m/s for a projectile that has a weight of 0.7g, the weight of a pellet according to the internet.

That sounds about right for a 0.20" bore pellet to me.

Building a dart that small is always going to be a challenge though. That's not to say there's no way it can be done, but I wouldn't fancy having to do it.
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Unread postAuthor: THUNDERLORD » Tue Jun 09, 2009 7:39 pm

No time to read entire thread now :( ,
But I've always thought a spent co2 cartridge with the nozzle and some of the body sawn/grinded off, then fill the tip with lead.
Nice hollow base, full metal jacket.(?)

"Buchanon" brand wire nuts have a version with two fins so electricians don't hurt there 'wittle fingers screwing them on wire connections.
Thing is the two fins appear spiraled!
They fit snuggly into 5/8" thin brass tubing too.
Seems like a potential skirt to create a nice diabolo or dart tail.
I'd probably still attach fleches just trimmed down to same diameter if used on arrows. 8)

EDIT: They're red and marked "B-2" or "B-3" (Dang memory, beer levels low...But will have to stay that way) :P :( :) 8)
Dang, Edit2: 5/8" brass tubing fits extra snugly into 1/2" brass nipple pipe.
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Unread postAuthor: jackssmirkingrevenge » Tue Jun 09, 2009 11:28 pm

It's worth noting that when fired from rifled barrels, fin stabilised saboted rounds have "slipping" driving bands to ensure spin is not imparted to the projectile because it would hurt accuracy.
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Unread postAuthor: psycix » Wed Jun 10, 2009 3:38 am

Also, sabot rounds are usually kinetic penetrators and thus you wouldn't want to waste you KE in spin.
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 10, 2009 10:40 am

psycix wrote:Also, sabot rounds are usually kinetic penetrators and thus you wouldn't want to waste you KE in spin.


You're still losing some by drag stabilisation flying through the air, it's mostly the lack of friction in the barrel that makes the smoothbore a better choice in terms of velocity for tank guns.
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Unread postAuthor: john bunsenburner » Wed Jun 10, 2009 11:01 am

God damn it this is complicted!!!
So spin is good, but doesn't help accuracy however it does help as it only compensated for areodynamic flaws(and there will be lots of them in my darts...:'()

Now rag, you have this awsome way of making complicated things sound alot more complicated than they really are...atleast some times. This part sounds important:
This means you have to balance the gyroscopic spin of the projectile against the drag on the fins. That is, you need to try and avoid spin rates too high, or ones which are naturally unstable to avoid causing issues with the stability of the projectile.

Mind explaining what gyroscopic spin is, how i ballance it to the drag of the fins, how i calculate the drag of the fins and how there can be a naturatly unstalbe spin rate(does it simply sling at the wrong speed or is it more than that?

Thanks for all your help!
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Unread postAuthor: ramses » Wed Jun 10, 2009 11:12 am

Oh poop. It looks like we're back to the argument about spinning darts. APFSDS rounds use slipping bands so as not to waste KE in spin. They use fins to make sure that the round hits the target point first. If that doesn't happen, it won't penetrate, so the extra few 10's of m/s won't matter.

In the "to spin or not to spin" argument, I side with rag. aerodynamic flaws will kill accuracy, but if the darts spin, the flaws will "cancel out" and the dart will corkscrew. While that sounds bad, it is better than the round arcing unpredictably.

I think discussion has gotten rather far away from "lathe turned pellets" :D
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