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Spud Showdown

A place to ask general spud cannon related questions.
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Unread postAuthor: Jared Haehnel » Fri Mar 21, 2008 6:55 am

Sabot isn't quite the right word, but it's the closest equivalent I can think of. Where sabot implies it supports the round, the basic principle matches JSR's idea from here, but refined a bit :


Just place a lip around the end of the barrel that stops the sabot and the dart continues forward.
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Unread postAuthor: Ragnarok » Fri Mar 21, 2008 8:49 am

MaxuS the 2nd wrote:How are you going to sight the plate at 400m, Rag?

That's a bloody long way to be looking for a (I'm guessing relatively small) steel plate.

Not that small a plate, but not vast, so I considered the problem.

The scope I have is the popular 3-9x40 - not really ideal for long range shooting, but usable.

Of course, this adjustable magnification means I can either see a fairly wide angle, or at full magnification, see the target large enough I can take aim properly.

I'll look into it. Worst comes to worst, I could paint the plate bright red, or tape a piece of paper over the front to make it more distinct.

@Jared: That idea isn't that great if I'm honest. The sabot, although it's going to be as light as possible, at the speeds of around 900 fps is still going to carry enough energy that a "retaining lip" isn't really an option.

Usually if any design requires a part with more than a few joules of energy to be stopped quickly, then it's going to damage itself, and it's not the right design.

Also, as it's going to have full calibre fins, the dart itself would get caught or damaged by the lip.

My plan for slowing the sabot is much more tricksy than that. I know I wasn't going to explain how I was going to do it yet, but... I'm going to fit a new attachment to the twist lock system on HEAL's muzzle, that uses the launcher's own power to form a vacuum behind the sabot. But this is only part of my plan.

But what I'm really pleased about is that when an dart & sabot are fired through the attachment, the speed of sound around the sabot is physically changed. A sonic shockwave forms ahead of the sabot, and that's used to generates transonic drag forces, which combined with the vacuum behind the sabot, slows it sufficently that it's separated quite significantly from the dart.

The dart itself is only minimally affected by this, because it's design is built to cope with subsonic, transonic and supersonic drag as best as possible.

And because the attachment uses some of the energy from the muzzle blast to generate this vacuum and transonic drag, then bleeds off the rest it's two birds killed with one stone.
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Unread postAuthor: potatoflinger » Fri Mar 21, 2008 10:15 am

spudfarm wrote:potatoflinger: hybrid's...

But they don't really count because they aren't just a pneumatic or just a combustion. (hence the name hybrid)
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Unread postAuthor: DYI » Fri Mar 21, 2008 10:19 am

Ever notice how this thread has veered a bit off topic? :roll:
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Unread postAuthor: Jared Haehnel » Fri Mar 21, 2008 10:31 am

While there is no harm in trying I just can't see you getting that kind of accuracy out your cannon. Let me give you an example. 3 to 5 moa is what my m1 carbine is rated at. That is, give or take a few, 3 to to five inches groups at 100 yards. Now that rifle uses the .30 carbine which is only effective out to 200 yards. The bullet travels at give or take 1800fps.
Even with your dart you'll have to arc the shot to make it. I'm not sure you'll have enough adjustment in the scope to make it arc as much as you want. I doubt the round will be traveling faster the 1000fps. Really a spud gun makes more of an effective artillery piece then a rifle.

I just realized this post sounds kind of "know it all-ish"...its not meant to be and I'm not looking to disprove anyone. I realized that Rag and many others are way more experienced in the ballistic department then I but I still think we've over looked an important point 8)
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Unread postAuthor: Ragnarok » Fri Mar 21, 2008 9:09 pm

It does need a bit of arcing - I'll need to measure the exact muzzle velocity and drag coefficient to know for sure, but it's in the realm of whole degrees. Only a couple of degrees mind you, but still quite a bit. I don't know if the scope's internal adjustments will suffice (They might just, but I've never checked), but I've designed a scope mounting that can be used for larger adjustments if I have to.

Now, your .30 carbine is closer to a pistol round than a rifle round. It's quite lumpy, and not particularly aerodynamic - also, the large powder charge is going to create a large muzzle blast, and thus some precession (if that's the right word) in the round.
Relatively high drag affects the stability of rifled rounds at long ranges.
It's not a round that you'd expect to be particularly accurate.

Drag stabilised rounds can easily be as accurate, if not more so than even pretty decent rifle rounds. I don't see why it wouldn't be possible to coax 3 or 4 MOA out of the rounds I'm making - because there's a nice , and rather sophisticated laser cutter at my old school that's used for cutting plastic sheets, and I'm still on good terms with the tech teachers... I'll say no more.

Although my initially lower velocity requires a higher arc, the better drag characteristics of the dart really shine through at long distances, helping it to maintain a smoother arc.

Believe it or not, at 400 metres from the muzzle, the dart from HEAL will actually retain more energy than the bullet from your carbine - 370 Joules, compared to about the ~260 Joules I estimated from your carbine (given I don't have an exact Cd, it might be out a smidgeon).
Given I started with about a third of the energy, then end up with ~50% more, you can see how much low drag projectiles help.
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Unread postAuthor: rcman50166 » Sun Mar 23, 2008 7:49 pm

wow, I leave for a day or two and my topic is hijacked!!! :shock: Oh well I have updates that will bring it back on track. I've finally taken pictures of the new and improved "General". I think you'll like them. Just gimme a sec to put up pictures.
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Attachments
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Unread postAuthor: bigbob12345 » Sun Mar 23, 2008 8:14 pm

Wow you definatley have made some improvments
there is a fan in each chamber right?
god that 10ft barrel will increase velocity and ensure the loss of elitesniper
which is too bad
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Unread postAuthor: Jared Haehnel » Mon Mar 24, 2008 8:04 am

That's a very nice gun... I'm excited to see how it preforms

Sorry for Hijacking your topic but this is really interesting to me.

There is no doubt that the .30 carbine was not developed to be an accurate round. I used it as an example because I've got a pretty go idea of what the ballistics of the round is. There is no doubt that .30 round would have lost most of its umph when it hits the 400 meter mark. And the shooter would have to use an considerable arc.

How ever my point wasn't to say .30 carbine would out preform your dart. With my experience in firearms and staring at various bullet ballistic charts. I just think that shooting a dart no matter how low of drag it has it will still drop at a constant rate. Do to gravities influence. Given the dart will still slow considerably before reaching the target you would have to be very careful and very accurate in figuring out how high to arc you shot to that it impacted at exactly the right moment during its flight. Given the fact that the only time a bullet is "on target" is at the point of impact.

I won't doubt that the dart will still retain enough energy to do some damage but you'd be better of starting at closer ranges. Keep in mind that even at 4moa your group sizes will be in the order of 16 inches at 400 yards. Given that fact that four hundred meters is a bit further then four hundred yard your group sizes will be bigger.

I'm pretty sure scope is out of the picture you will have to figure out some other way to accurately aim the gun at those ranges. Perhaps 200 yards or three hundred yards you'll have better luck.
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Unread postAuthor: rcman50166 » Mon Mar 24, 2008 12:08 pm

I may have a new target. A car. My friend works for the local fire department and they have a junker they're going to burn as a training excersize. He says I may be able to get a few shots in before they send it up in flames. You guys think I'll be able to get through it if I aim at the doors?
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Unread postAuthor: Jared Haehnel » Mon Mar 24, 2008 12:14 pm

I think it would be a bit of a stretch...If you held the muzzle right up against the door maybe but it would have to go through a lot of stuff. With your nail probably not... but it would be worth a shot to see how far it would go.

One door probably but two doors no...not a chance....thats my vote anyway....
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Unread postAuthor: rcman50166 » Mon Mar 24, 2008 4:55 pm

*sigh* where's the depleted uranium when you need it... Oh well, I'm still going to shoot at it. It's too good of an opportunity to give up. I'll just keep it out of the video. How bout a cinder block target? The ammo is a concrete spike. Would I be able to demolish that? Remember, my C:B ratio is approaching 1:1.
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Unread postAuthor: Jared Haehnel » Tue Mar 25, 2008 6:21 am

Is it a solid block or it is hollow...
I would say yeah it would at least break it up but probably wouldn't punch a hole through it.
Try Pyrex as ammo, basically sawdust and water mixed together then frozen...
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Unread postAuthor: Novacastrian » Tue Mar 25, 2008 6:27 am

http://www.spudfiles.com/forums/files/muzzle_203.jpg

That is a virgin if i have ever seen one :oops:
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Unread postAuthor: LikimysCrotchus5 » Tue Mar 25, 2008 10:40 am

Jared Haehnel wrote: Try Pyrex as ammo, basically sawdust and water mixed together then frozen...


I think you mean Pykrete.

Nova:

I see your mind is in the gutter again, but you are correct. :)
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