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Small bore vs. big bore - bullet performance

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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Small bore vs. big bore - bullet performance

Unread postAuthor: FighterAce » Mon Mar 22, 2010 3:00 pm

I've been making nail darts for about 1 year and never really complained about the 10cm grouping at 10 meters. But now I'm looking towards 10cm grouping at 100 meters and my old darts just cant fly so good.
My barrels are 12.70mm in diameter and shoot nail darts with a 7cm nail.
I've done some experiments that prove my theory that the longer the tail (behind the nail) the more stability you get out of the cone shape nail dart. For the test I used a 16mm bore with much larger and longer tail. Its center of gravity is more towards the tip then the dart for the 12.70mm bore and that makes it fly much more accurately.

Heres some info.. I'm going to be shooting out of a piston valve air gun, barrel length will be 80cm, pressure 400 psi with about 60 cubic cm of air. For consistent opening I will be using a spring powered hammer that will depress the pilot schrader and trigger the gun.

The smaller the bore, the less "pushing" power it has on the projectile but the smaller diameter bullet for that barrel can move through the air more efficiently. But what if you got more power behind the bullet with slightly more drag - less efficiency? And what differences in power/speed would you encounter down range with the 2 darts?


Here is a comparison between the darts and barrel sizes (16mm on the top, 12.70mm on the bottom)
Image


The smaller dart flies pretty good but it aint gonna do it at 100 meters. On the other hand the bigger dart flies excellent but I'm worried about drag messing with power at such long distances.
I'd need to choose between the 2 for my next ammo cause I'm running pretty low right now and need a good long range bullet for my next gun.

I'm not an expert on aerodynamics or have experience shooting long distances so I really need some help on this one.

btw. I ran the numbers through GGDT and I would get some extra Jules and fps but I'd really need some real world data
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Unread postAuthor: jackssmirkingrevenge » Mon Mar 22, 2010 3:45 pm

10cm grouping at 100 metres is simply not something you're going to see with homemade darts. It's difficult to do using a commercial air rifle with a rifled barrel, scope and shooting good pellets at 850 feet per second. To do so with inconsistent ammunition and a smoothbore barrel is a little too ambitious in my opinion.

Another problem with your darts is that they are not centred, so it's anybody's guess which way they will be pointing as they leave the barrel. Don't want to sound doom-and-gloom but just trying to give some realistic input.
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Unread postAuthor: FighterAce » Mon Mar 22, 2010 4:03 pm

Alright not rly 10cm at 100 meters, I guess I was exaggerating a bit there. Perhaps at 50 meters?
There are quite a few methods to make the dart centered. You can use sabots, another cone to act as a centering device and add fletchings to it for spin, small fins to center the dart and make it more stable (in theory) and some others witch I cant remember at this point.

These darts can be made consistent with a number of jigs and good quality nails with consistent weights.
Rifling isnt the problem, I could easily melt it into the barrel but I dont think nail darts are very suitable for rifling because the nail tip could catch the groove and spiral out of control.

Normally after loading I give the breech a solid tap so the nail drops onto the bottom of the barrel so its relatively consistent in its position without any centering device. I'm pretty sure that as the dart accelerates inside the barrel, it centers itself but still, thats just a wild guess
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Unread postAuthor: Hawkeye » Mon Mar 22, 2010 4:16 pm

I like the idea of a field point for an arrow with a bucktail as a stabilizer. Make it as close to a commercial dart as possible.
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Unread postAuthor: jackssmirkingrevenge » Mon Mar 22, 2010 4:28 pm

Might I suggest something like this?

Image

You could easily make a reasonable quantity fairly quickly with a jig and some superglue, and you have the advantage that it will always be centred in the barrel. Also, bigger barrel diameter means more force accelerating the dart but once it leaves the barrel, drag tapers off dramatically as opposed to the full barrel area of the cone tail.

Another option (though perhaps not so easy outside the US) is to buy some proper flechettes ;)
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Unread postAuthor: FighterAce » Mon Mar 22, 2010 4:53 pm

I've seen this before and I really dislike the design, it just doesnt make sense to me. I can tell you right now its very unstable. The whole concept of darts is to have a light tail extend back away from the heavy tip. Like this you have a light tip and a heavy tail. Chances are, the dart will flip in mid air and fly with its fins forward and the tip in the back.

I would change the design to make it something like this...
Image[/u]

The fins would be connected with a thin strip to the nail so most of the nail is as far forward as possible. While thinking about this I realized a pretty big technical problem. How would you connect the wood to the nail?
I wouldnt have a hot glue gun or even electricity out in the field, super glue is just too damn expensive in such quantities, cord would be too weak and probably slip off and making slits in the nail for the cord is just impossible without a lathe. Not to mention the hassle of putting in the dart and then the foam disc. With all those things, the small connecting pieces of ply could just snap like dry twigs under the pressure.
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Unread postAuthor: Hawkeye » Mon Mar 22, 2010 4:54 pm

You can also buy teflon tap washers that would work as a seal and for centering the nail in the barrel.
Readily available sizes are a slide fit in half inch copper.
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Unread postAuthor: jean » Mon Mar 22, 2010 5:48 pm

hi
what you need is not a dart in you design.
better is full bore bullet like a drop or an other aerodynamic form.
center of gravity of the dart is very important. if the cog is to much in front of the dart -- the projectile begins to tumble if the velocity go down rapidly after a couple of yards.... gravity will pull the front of the dart to ground but not the end..... ah my english so bad i feel rag or jsr will tell you the background of pellet/cog and long distance.....

google search give you many links
search --- diabolo(pellet) cog and ballistic curve

standart flat 10m air gun pellet have cog much in front in an ballistik shoot it will tumble after ~60m (easy to controll with paper target.)
it comes from the to much forward cog ...simply not made for long distance...

so if you will longe range go with aerodynamic form of projectile and well balanced out cog

google ----->drag coeficient
you cone end is not the best (see on wikipedia)


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Unread postAuthor: FighterAce » Mon Mar 22, 2010 6:24 pm

If I shouldnt put the cog in front, where should it be? I'm kinda confused here. Shouldnt the cog be at least 60% the length of the dart away from the tail end to achieve stable flight? Its not hard to change the cog with those darts, I just need to know where to put it :D Also, what other forms would you recommend? The drop would be kinda hard to make...

btw. what about combining the cone and drop design?

Thanks for the reply
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Unread postAuthor: jean » Mon Mar 22, 2010 6:42 pm

yes try it out ...test it out in a wide range maybe 10-60% from the tip it depends also of the form
ah and more mass in relative to the surface give you a better performens at longe distance so go with the big diameter
take a look on airplane wings and her streamlined form
also a lite spin will help so the spin can compensate a non perfect projektile

making a mold i think is the best way to make accurate projectile

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Unread postAuthor: Mr.Sandman » Mon Mar 22, 2010 7:10 pm

Well in my current project of LRR I'm going for an effective range of about 450 yards.

With my prototype I noticed that darts arent very effective at longer ranger because of their light weight they are easily effected by wind. I suggest that you make a dart that consists of a bullet shaped nose a stem and then the fin.
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Yeah, it's that important.
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Unread postAuthor: 245Tommy » Mon Mar 22, 2010 8:06 pm

Make sure the end of the cone and barrel is cut square and you could try adding angled fins to correct yawing and the dart's inconsistencies.
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Unread postAuthor: jackssmirkingrevenge » Mon Mar 22, 2010 11:57 pm

I'll admit it's not the most aerodynamic of designs but it was just an example.

FighterAce wrote:The whole concept of darts is to have a light tail extend back away from the heavy tip


It's actually slightly more complicated than that, it's to do with balancing your centre of gravity with centre of pressure.

Have a careful read through this, the diagrams explain the dynamics of the parameters involved very clearly.

How would you connect the wood to the nail?


Image

:D :D :D

At the end of the day your cone dart has an extremely poor ballistic co-efficient, it has a "massive" frontal area - the full diameter of the barrel - but very low weight, meaning it might be impressive out of the muzzle but slows down very quickly.

If you want a good full-bore projectile, it will have to be heavier. Maybe find some pipe that fits exactly into your barrel and fill the nose with lead, something along these lines...
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Unread postAuthor: Technician1002 » Tue Mar 23, 2010 1:30 am

Using a disposable wadding on a larger bore works well for golf balls. I launched some with the GB barrel and then launched some from the 2.5 inch cannon using plastic pop bottle sabots. At the same pressure there is a noticable difference. The larger barrel requires a larger valve. You don't gain anything screwing a 2.5 inch barrel on a 1 inch sprinkler valve.

I am in the process of building a 2.5 inch valve for a 2.5 inch cannon. It will be used to launch some golf balls and other things. A 3 inch cannon is on the drawing board. The 2.5 inch is a test bed for bumpers and other materials testing before the 3 inch build.

As soon as spring cleanup is finished (gardening, gutters, greenhouse etc) then the cannon will contine that I started last fall.
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Unread postAuthor: dewey-1 » Tue Mar 23, 2010 8:00 am

This posting shows some dart ideas.
Ready made ones by Ramset or Remington.

http://www.spudfiles.com/forums/nail-da ... 19586.html

Have a look here for center of pressure calculations;

http://www.spudfiles.com/forums/unstabi ... 19849.html
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