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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: inonickname » Fri Apr 01, 2011 3:27 am

jackssmirkingrevenge wrote:
inonickname wrote:1222.01m with a man cocked (in one stroke no less) and portable, probably about 30" powerstroke weapon.


Obvious innuendo aside, any idea how fast that was travelling?

The thing with arrows is that sabots can get a bit complex, and really you should be firing it out a 1" or so bore at least.

LeMaudit wrote:I've cut a whole range of materials with my cheap Sears band saw, from plastic to soft metal. The trick is to use a variable speed one. At the lowest speed, I go through a 1 inch aluminum rod like butter. But VERY slowly


"Like butter" means quickly and easily - unless you meant "like I was cutting it with a sharpened stick of butter" :D :D :D


Jack- the PSE omen is generally regarded to be the fastest bow out of the box on the planet, with an IBO of ~366 fps. IBO is a best possible scenario speed, and it's not really very safe to shoot with a lighter arrow for more speed (IBO is a 70# draw, 5 grains per pound of draw, at a 30" draw length). You CAN shoot with a lighter arrow than 5 grains per pound, but it's potentially dangerous and damaging to the bow. I'm guessing they'd have to find a happy medium between mass and velocity for optimal distance, so a figure of 300-350 fps would be reasonable. We should be able to exceed this easily- a larger bored combustion with a sabot should be able to double that velocity.
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Unread postAuthor: Ragnarok » Fri Apr 01, 2011 8:41 pm

jackssmirkingrevenge wrote:The Grand Slam (1) would approach the speed of sound by gravity alone, so it's a good design to emulate :)

It achieved that by virtue of its mass and sectional density, not any particular aerodynamic mastery.

If you figure it, drag increases with the cross sectional area, mass with the volume... hence, Square/Cube law, bigger objects have a higher terminal velocity than smaller objects.

~~~~~

Anyway, if it wasn't now on (likely permanent) hiatus, the dart project I had was working out with a projected maximum range approaching 4 miles. It was somewhat dependent on how well the PTFE coating on the darts could actually reduce drag, but the results were coming out well.

The VLCs were beyond four nines, which gave them a terminal velocity faster than I could fire them - which would have been great, if there was actually anywhere I could have used them properly.
(Combine that with the fact I've been going through a non-spudding phase for the last few months, so I've not exactly been that proactive overcoming setbacks...)

However, I do have to speak up on the safety issue. If a long range projectile is fired, then it has a maximum range of "out of sight" and will carry lethal energies a hell of a lot further than an inefficient projectile that'll lose most of its energy after a hundred metres.

For that reason, many of my "damage" projectiles are deliberately poor aerodynamically. It's the difference between an accident where things within 30 metres are at risk and one where things within 3000 metres are at risk.
I wouldn't recommend using a longer ranged projectile than you actually need to.
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 02, 2011 12:18 am

LeMaudit wrote:I'll make you an ammo to whatever design. A short project that I think I can manage on my lathe and mill. And you'll shoot it :D


I was hoping this was the sort of cooperation this thread would inspire.

irisher wrote:does anyone have any info on 3 vs 4 fins for dart rounds?


It should work well anyway, remember that many crossbow bolts only have 2 fins ;)

inonickname wrote:We should be able to exceed this easily- a larger bored combustion with a sabot should be able to double that velocity.


The BL-520 is quoted as doing 517 fps with a 100 gram projectile. I reckon that with a 35 gram or so arrow in a lightweight sabot would easily go beyond 750 fps.

Ragnarok wrote:It achieved that by virtue of its mass and sectional density, not any particular aerodynamic mastery.

If you figure it, drag increases with the cross sectional area, mass with the volume... hence, Square/Cube law, bigger objects have a higher terminal velocity than smaller objects.


I disagree, aerodynamics is certainly a factor. Otherwise, the 12,000lb HC bomb shape (see attached diagram) would have been adequate. The Tallboy's thicker steel body of course helps with penetration once it hits the ground but not with acceleration the HC and Tallboy have similar weights and diameters and therefore cross-sectional density.

However, I do have to speak up on the safety issue. If a long range projectile is fired, then it has a maximum range of "out of sight" and will carry lethal energies a hell of a lot further than an inefficient projectile that'll lose most of its energy after a hundred metres.


I mentioned this in the original post, it is essential that the shooting area is deemed to be completely clear for several kilometres.
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Unread postAuthor: MrCrowley » Sat Apr 02, 2011 12:22 am

The BL-520 is quoted as doing 517 fps with a 100 gram projectile. I reckon that with a 35 gram or so arrow in a lightweight sabot would easily go beyond 750 fps.

My piston hybrid will do 520fps average velocity with a 100g projectile over 70m, muzzle velocity is closer to SOS. That should be adequate enough...I hope :shock:

Speaking of weights, I was thinking a dart closer to 100g would be much better than something that weighs only 35g. Last time I played around with this stuff on HGDT, 100g projectile seemed to be pretty damn good when it came to long ranges (assuming you have a very low C.d. projectile).

Perhaps we should have some more discussion on optimised projectile weight (for a given bore and range of projectile C.d.).
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 02, 2011 1:17 am

MrCrowley wrote:Speaking of weights, I was thinking a dart closer to 100g would be much better than something that weighs only 35g. Last time I played around with this stuff on HGDT, 100g projectile seemed to be pretty damn good when it came to long ranges (assuming you have a very low C.d. projectile).


It's not so much about weight but density.

Let's take two darts as examples, ignoring the fins. One is a wooden (say beech, 0.8 g/cm<sup>3</sup> density) dowel, 10mm in diameter. Another is a steel (8.0 g/cm<sup>3</sup> density) rod, 2mm diameter. Both are 20cm long.

The steel dart weighs less than half the wooden one, so will be fired faster. However, it has a sectional density of around 160 g/cm<sup>2</sup> - 10 times greater than that of the wooden dart - meaning that not only will muzzle velocity be greater, but so will velocity retention.

In short, it needs to be as small diameter as possible, while being as dense as possible, as well as being strong enough to resist being deformed by acceleration forces in the barrel.
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Unread postAuthor: LeMaudit » Sat Apr 02, 2011 8:53 am

I've grabbed some various material from the spare box and my precision scale to have a feeling about the size of the dart.

MrCrowley, you said your barrel diameter was 45mm right? Or is it in inches, 1 3/4” ? Please give me a very precise value. Would be a shame the dart don't fit the barrel at the end :lol: Maybe I can find the same barrel locally.

Any suggestion about the material to use? I guess metal would be too heavy to stay in the 100gr range. Unless it's mostly hollow. I've weighted some PVC and UHMW. The latest seem to be light and strong, maybe a good choice? Like those UltimateSpudGun rounds.

A UHMW 45mm massive rod with a weight of 100gr would be about 70mm long. I'm guessing with removing material on the tail to shape the fins, and maybe adding a bit of weight on the nose, a 100gr UHMW dart could be 100 to 120mm long. The low surface friction of UHMW is also a good reason to use it.

There's always the sabot solution. To keep the dart small (metal?) with a significant density, Surely the best choice in term of efficiency. But there's the issue of finding it after the launch :? Unless it's long enough, like a small arrow.

I'm not so used to machine plastic. I've done only acrylic do far, and UHMW. I can't say machining UHMW way easy :-( I've read on machinist web sites that Delrin is a good choice for easily lathe/mill machining. But a web search gives some weight density for Delrin about 50% move than UHMW. Just a bit lighter than PVC. It seems to be strong enough with low friction (Wikipedia source) somehow close to UHMW. Anyone with some experience machining plastics?
The biggest chunk of plastics I have are about 1 inch thick, but there's a shop that sell all kind of plastics in town, so I could have a look for something specific.

I share Ragnarok's concern about safety. This beach of yours MrCrowley, can you be 100% sure there will be no one there when you fire the dart? Do you have a clear view all the way?
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Unread postAuthor: Crna Legija » Sat Apr 02, 2011 9:39 am

why not go small scale, that way you get the same theory and wont risk safety of randoms going for a nice walk by the beach.

lets hope MrCrowley doesn't get beached az :D
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Unread postAuthor: Technician1002 » Sat Apr 02, 2011 10:16 am

I started to test golf ball range starting with low pressure with one of my launchers when I had the chance to use a large farm. The test ended when we got rained out at 50 PSI. :( Hopefully someday, I'll be able to continue and work up to 100 PSI. We were able to find the golfballs shot with 50 PSI air.
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Unread postAuthor: Labtecpower » Sat Apr 02, 2011 11:35 am

That's about the same range i'm getting with my gun.
I used a slightly less aerodynamic projectile however.

(a golf ball with a cardboard tube strapped to the back)
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 02, 2011 1:11 pm

Labtecpower wrote:(a golf ball with a cardboard tube strapped to the back)


According to NASA it seems it's a better aerodynamic shape.
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Unread postAuthor: Labtecpower » Sat Apr 02, 2011 1:20 pm

I think a sphere is more aerodynamic than my bullet, as it tumbled quite a lot. It made a sound similar to a jet fighter.. the sound of inefficience :roll:
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 02, 2011 1:29 pm

Labtecpower wrote:I think a sphere is more aerodynamic than my bullet, as it tumbled quite a lot.


You mean like the attachment? Technically it should fly straight...

It made a sound similar to a jet fighter.. the sound of inefficiency :roll:


On afterburner, yes... but nonetheless impressive ;)
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Unread postAuthor: Labtecpower » Sat Apr 02, 2011 1:37 pm

I made it exactly like the diagram. It spiraled and tumbled for the first 100 meters.

The sound it made was very cool, but not good for distance shots.

My father was very curious how the cannon performed. He went with me, and said: "I'll recover the projectiles for you" When he saw how far the things went, he wished he didn't said it :D
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Unread postAuthor: Fnord » Sat Apr 02, 2011 1:42 pm

Has anyone every considered shooting tungsten electrodes for a tig welder? You're not going to get much more sectional density than that.

Tungsten is 0.71 pounds/cubic inch, whereas lead is only .42.

Until I heard about the weight limit they put on USPS flat-rate boxes, I always wanted to fill one up with lead or tungsten and send it to the other side of the country , lol
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Unread postAuthor: jackssmirkingrevenge » Sat Apr 02, 2011 1:51 pm

Fnord wrote:Has anyone every considered shooting tungsten electrodes for a tig welder?


Good suggestion, a 1/8" wide 7 inch long tungsten rod would weight just 25 grams and would be a fantastic projectile if fitted with fins. As most will know, tungsten cored projectiles are extensively used in armour piercing applications due to both high density and hardness.

As such it would also be a tremendous penetrator which would not need to be going very fast in order to go clean through someone, so again safety and a clear range is paramounts.

Some notes on optimised projectiles and range in the real world, courtesy of fr frog talking about the M829 APFSDS round:

Lets see now -- KE (ft lbs) = (W * V²) / 450436. That gives us (65,870 * 5480 * 5480) / 450436 which boils down to (hold on to your hats!) 4,391,500 foot pounds (that's right 4.3 MILLION ft lbs). Sectional Density = W (in pounds)/D². That gives us 9.41 / .6 * .6 which equals 8.4. (That's eight point four--and you thought your 220gr .30 cal bullet had high sectional density at .343 (that's "point" 343.) The newer M829A3 round throws a 22.2 pound dart at 5200 f/s for 9.2 MILLION ft/lb.

As an interesting side note, when fired at 55 degree elevation, the A2 will travel 113,111 meters (123,630 yards or roughly 70 miles) and reach a height of 44,073 meters (48,171 yards or almost 27 miles). You are NOT safe if someone aims one of these at you. The accuracy of the dart is sub-MOA and the stated effective range is about 3000 meters (3270 yards). However, first round killing hits have been obtained out to about 6000 meters (6540 yards).

Sharp-eyed readers may note the 55 degree elevation for the maximum range of this projectile. While spin stabilized cylindrical projectiles reach their maximum range in air at angles of between 25 and 35 degrees (45 degrees in a vacuum) the "dart is fin stabilized and not spun (the 120 mm gun is a smooth bore) and this gives it its maximum range at 55 degrees elevation--the fins keeping it point first through out its trajectory and providing some lift. Click here for a discussion of projectiles fired at extreme elevations.


Note also the discrepancy between optimum launch angles between spin and fin stabilised projectiles.
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