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Fin stabilized and/or sub-caliber ammo?

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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Fin stabilized and/or sub-caliber ammo?

Unread postAuthor: Alex345 » Thu Dec 10, 2015 12:35 pm

I abandoned my last project because of a long planned vacation trip, but now that I'm back and fully recharged i feel like i just have to finish her! It's a single shot pneumatic 30mm cannon thats meant to be fired like a rifle. The barrel is a 1m long piece of aluminum profile tubing, so it's a very nice and seamless smooth bore. I mainly want to aim for range and accuracy.

I've decided that making self stabilizing ammo would be far more feasible than actually making a rifled 30mm barrel since a PVC barrel is not an option... Rifling the aluminum barrel would weaken it's structure and the projectiles driveband would have to be made of an even more ductile material if i don't want it to rip the barrel apart at higher pressure shots.

My first question would be; full bore or sub-caliber? If I understand correctly, a sub-caliber projectile (APFSDS) is only used for it's extreme penetrating capabilities. Applying the full force of a cannons bore on a spot thats only a fraction of that. I bet that they are also faster and suffer less from drag and friction because of their shape.

Problem is, they are only accurate if the sabot is very precisely made. And i have no machining or 3D printing capabilities... Only basic tools; yet i already have a decent idea in mind.

Or would it just be better to stick with a full-bore fin stabilized projectile?

If thats the case; enter question number 2!

I've noticed that a 2" long 3/4 nipple fit's the bore almost perfectly, i was thinking about buying the cheapest arrows i can find and cut off the fletching part and a few inches of shaft. I would pour lead into the nipple and drill out a hole for the shaft and epoxy it in place.

I'm just worried that when the valve opens the fletchings would get blown off or something. Or that arrow fletchings are too soft/weak to stabilize such a heavy projectile?

I'm also very curious about any previous attempts made on this forum on making fin stabilized ammo. Or even other methods ( My craziest idea was a round containing a CO² cartridge with angled jets to be punctured upon firing to cause a spin).

Oh and did they ever shoot that Miniboy projectile? I remember following that topic a long while ago...
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Re: Fin stabilized and/or sub-caliber ammo?

Unread postAuthor: Ragnarok » Tue Dec 15, 2015 7:59 pm

Alex345 wrote:I mainly want to aim for range and accuracy.
Do you have specific numbers?

I've got a on-and-mostly-off project to achieve something similar with a 20mm calibre pneumatic. The original objective was to be able to hit and penetrate a 40cm diameter 6mm mild steel plate at 400 metres (equivalent to one milliradian), although the reality is that the testing is all having to happen at a rather shorter distance.

If going for range, I'd advise the sub-calibre path, as it leads to a more aerodynamic and faster (due to the lower weight) projectile. Here's some older prototypes of the projectile design I've been working on...

Image

... but the current version is a three-finned design (with bevelled leading edges on the fins), as reducing the number of fins reduces the skin drag on the projectile quite heavily (but three is still enough for stability). It has bore-riding fins, which allow the sabot to just be a simple 20mm plastic disk behind the projectile (thus drastically simplifying the problem of sabot separation, which is why they normally require such precise manufacture)

That may be of use to you as a concept.
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Does that thing kinda look like a big cat to you?
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Re: Fin stabilized and/or sub-caliber ammo?

Unread postAuthor: jackssmirkingrevenge » Tue Dec 15, 2015 8:42 pm

It has bore-riding fins, which allow the sabot to just be a simple 20mm plastic disk behind the projectile (thus drastically simplifying the problem of sabot separation, which is why they normally require such precise manufacture)


Won't work... projectile CG is ahead of the fins, so the projectile will likely tip forward in the barrel and destroy itself. Trust me...

Oh and did they ever shoot that Miniboy projectile? I remember following that topic a long while ago...


Nope, looking at you @MrCrowley /u/MrCrowley dammit can't we summon folks on this forum?
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Re: Fin stabilized and/or sub-caliber ammo?

Unread postAuthor: mark.f » Tue Dec 15, 2015 11:07 pm

jackssmirkingrevenge wrote:Nope, looking at you @MrCrowley /u/MrCrowley dammit can't we summon folks on this forum?


Jack has an Instagram? :-o :D

#sectionaldensity #casioexilm #eurosuck
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Re: Fin stabilized and/or sub-caliber ammo?

Unread postAuthor: jackssmirkingrevenge » Wed Dec 16, 2015 7:19 am

mark.f wrote:Jack has an Instagram? :-o :D


Hah! I used instagram exactly once, and that was to add images of my favorite barmaid to my spank bank. Imgur uses the same system of user summoning though :wink:
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Re: Fin stabilized and/or sub-caliber ammo?

Unread postAuthor: Ragnarok » Wed Dec 16, 2015 6:04 pm

jackssmirkingrevenge wrote:Won't work... projectile CG is ahead of the fins, so the projectile will likely tip forward in the barrel and destroy itself. Trust me...

... or I could trust the real-world results. :P


The fins are a tight fit in the barrel, and naturally align the projectile with the bore (being more than strong enough to support a fraction of a newton a few centimetres ahead, given they have to survive kilonewton forces on firing). I've filmed different variations on these darts in high-speed a great many times, and they've always flown intact and stable.

I'll admit that interference fits probably wouldn't be a good idea for the shotgun penetrators you've been working on (where you've got forcing cones, varying interpretations of bore diameter, etc to worry about), and the whole design probably wouldn't translate well to really large bore diameters, but for a small-ish calibre pneumatic with a smooth seamless barrel it's a simple and effective design.
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Re: Fin stabilized and/or sub-caliber ammo?

Unread postAuthor: Alex345 » Thu Dec 17, 2015 3:31 pm

Ragnarok wrote:Do you have specific numbers?
God, i still need to find a decent way to pressurize the cannon to around 435-500 psi, completely forgot about that too... I think the valve can handle quite a bit of pressure but i wanna take it slow :)

I have nothing in mind numbers wise, i just wanna make a decent projectile and get the cannon up to pressure. It still needs an entire trigger mech, but it can also be fired manually in case the pressure source and ammo get done first. Once it can shoot, i can slap a scope on it and see if it's any bit accurate/consistent.

My current idea for an APFSDS is a piece of M8 or M10 threaded rod with about 2mm of material removed in the middle (Don't have a lathe but i might know somebody who does) and with two slots cut into the back making an X shape in which two metal plates will be inserted that also slide into each other to create a dart. And a two piece spindle sabot with 2mm grooves that fit into the part where the metal was removed to keep in place.

If this confuses you guys i can always draw it up in paint :)
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Re: Fin stabilized and/or sub-caliber ammo?

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Re: Fin stabilized and/or sub-caliber ammo?

Unread postAuthor: Ragnarok » Thu Dec 17, 2015 6:12 pm

Alex345 wrote:I have nothing in mind numbers wise

Well, if you've got any significant distance you may want to reach, knowing that would be a good start as it's then possible to work out a minimum velocity (assuming that you want a relatively flat trajectory, no more than a couple of degrees of elevation) and estimate how heavy a projectile the cannon can fire at that velocity.

That's largely where I started with my designs. I knew I wanted to a 400 metre range with about a 1.5 degree elevation (much higher than that and the barrel would be angled through the scope's line of sight), I needed a velocity of at least ~270 m/s.
From there, I could come up with a range of combinations of mass and ballistic co-efficient to evaluate designs by.
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Re: Fin stabilized and/or sub-caliber ammo?

Unread postAuthor: Alex345 » Fri Feb 12, 2016 4:50 pm

Well I've recently made one of these when i was bored.

round2.jpg

round1.jpg


It's sub-caliber, so i still need a way to shoot it out of my cannon
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