Bullet Aerodynamics

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Bullet Aerodynamics

Postby TodayIsTomorrow » Tue Oct 21, 2008 8:27 pm UTC

Alright Ladies and Gentlemen,

I fancy myself to be a bit of an inventor and engineer, but I've run into a snag. What I'm trying to do is design a rifle with a cartridge I design. The problem is that I can't figure out the aerodynamics of bullets to save my life. I know that the .50 BMG bullet and the .408 Chey Tac bullet are two of the best designs for long distance shooting. My question is WHY they are the best. I know they are heavy, I know their shapes have something to do with them staying supersonic at longer ranges than other rounds and I know that the shock of going from supersonic to subsonic will destabilize bullets to an extent.

So, to all you physics and aeronautics majors out there, I need some help. I need a program for the computer that I can model aerodynamics with. Something like a 2D wind tunel would be awesome. I've got some ideas for bullets of unconventional shape that I want to try out, but short of making a bunch of bullets and then firing them out of a perfectly good firearm (which is a REALLY bad idea if you have no idea what they're going to do when they exit), I'm kinda up a creek.

Also, if you guys could try and put into laymans terms how to figure out ballistic coeficients and drag, I would be most appreciative.

I hope this post makes sense to people other than myself. I'm wigged out on caffeine right now and I'm tearing what little hair I have out of my head trying to figure out why certain bullets work so much better for accuracy and range than others.

Cheers
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Re: Bullet Aerodynamics

Postby Alias » Tue Oct 21, 2008 8:30 pm UTC

why are you trying to build a gun?
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Re: Bullet Aerodynamics

Postby TodayIsTomorrow » Tue Oct 21, 2008 8:37 pm UTC

...why wouldn't you want to build a gun?

I like guns and I've got several ideas of how to do things better than the conventional designs of firearms. The best way to learn about how something works is to build one yourself. Plus, if it works and everything like I hope it does, I'll have a rifle that recoils like a .308, with the accuracy and range of a .50 BMG. Between a recoil absorbtion (I don't even know if that's a word...) system, a floating action, and the weight of the rifle, I should be able to achieve this.

So, short answer, I want to make a rifle that's better than the rest.
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Re: Bullet Aerodynamics

Postby Monty40xi » Tue Oct 21, 2008 9:09 pm UTC

A 2D flow modelling program wouldn't be enough for spinning cylindrical objects like bullets. You need a full-blown 3D modeller and the hardware to support it. At work we use a program called Flotherm, although that's aimed at moving air around stationary objects so it might not be the best for ballistics.

If you really do thing you've got great ideas for building better guns, my best advice is to get a job with a firearms manufacturer. Just don't go into full details on your ideas until you're getting paid. You just won't be able to produce things as reliably, safely, or affordably as a company could, or else the companies wouldn't exist in the first place.

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Re: Bullet Aerodynamics

Postby TodayIsTomorrow » Tue Oct 21, 2008 9:24 pm UTC

That's why I love my little Gullthulhu.

I was afraid that a 2D modeller wasn't going to be what I needed and I sure don't have the computing power to run a good 3d one. As for contacting a firearms manufacturer with my ideas, I simply don't have the cash to get them to prototype anything, nor do I have any contacts in R&D departments. As for getting a job with a firearms manufacturer, I've got until 2012 before I can even think of getting a job with anyone besides the Army, and Uncle Sam generally frowns on people in the services having a second job.

I suppose I'll have to put my ideas on hold for a while until I can obtain a job with an R&D department or I can get access to the tools I need for proper design work.
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Re: Bullet Aerodynamics

Postby Monty40xi » Tue Oct 21, 2008 9:46 pm UTC

4 more years in the Army? I can't think of a better way to start getting contacts in the firearms biz. Engineers for weapons companies love getting first-hand input from the folks in uniform - the upper leadership tells us what they think a new item should be used for, but it's the soldiers on the ground who tell us how it'll ACTUALLY be used (a friend of mine in the Army told me to stop sending them radios with external antennas, because they just cut those off). Maybe your CO knows about some new contract that could use a soldier's perspective, and you can start making your contacts there.

If you need a quick demo piece just to get an idea across, look for machining houses with rapid prototyping machines. They can turn a CAD design into a full-sized mockup very quickly. A non-metallic mockup, but a mockup nonetheless.
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Re: Bullet Aerodynamics

Postby Charlie! » Wed Oct 22, 2008 2:50 am UTC

Hmm, according to my google-fu (I don't actually know what you want, but I can help find it :) ), you may find these handy if you're designing a bullet:

http://bryanlitz.bravehost.com/BulDes.html
http://www.chuckhawks.com/bc.htm

Also, a third site I ran into ( http://www.lima-wiederladetechnik.de/En ... bullet.htm ) was touting the virtues of the .338 LM-105. There are similarities and differences between them and the Tac bullet that you might find interesting.
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Re: Bullet Aerodynamics

Postby Starside » Wed Oct 22, 2008 3:56 am UTC

One man designing a rifle on his time in his garage or similar is plausible way to create a new weapon, especially if you have experience with how they work. I should point out Barret, the manufacturer of a .50 cal sniper rifle was started this way: http://www.barrettrifles.com/about_history.aspx
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Re: Bullet Aerodynamics

Postby TodayIsTomorrow » Wed Oct 22, 2008 5:13 am UTC

Wow, learn something new every day. The fact that Barret started out so small and is now one of the best sniper rifle manufacturers, and a very respected name in the military gives me some hope. Now all I have to do is make a better rifle than Barret for cheaper. With the 8000+ dollar price tag that the Barret .50 BMG has, the cost shouldn't be hard to undercut.

Unfortunately, my government internet won't allow me to load the first two links you posted Charlie, but the third is absolutely mesmirizing me, mentioning some really odd ball rifles in the development of long rang accuracy. I'm loving it.

I'm also noticing that most of the really good bullets have what look to be ribs on them except for the .50 bmg. I was thinking about designing a bullet with a scaled type look with the notion that perhaps keeping the pressure wave traveling along smaller points will decrease the drag overall. Seeing those ridges on the bullets gives me hope that I'm correct in my thinking. I'm currently looking at a picture that's labelled "Three 8,5 mm bullets" and I'm seeing what looks like an exponential scale to measure the distances between the ridges of the last one. I'm really really curious why they would measure out the ridges like that. The bottom bullet is the LM-105 that apparently performs ridiculously well compared to the conventional .338 Lapua Magnum and the .50 BMG. Those ridges have to have something to do with that performance, though once again, I don't know what.

Once I'm back at my room, I'm going to load up those two links and see if they help me out. I'm now convinced that those ridges are the key to making a bullet that works well. I want to design something smaller than the .338 in diametre, but longer in the bullet. Anyone up for a .30 calibre bullet that's 3 inches long? I'll rake a peek at the bullet design links and see what comes of it. I'll post some designs and results if anyone's interested.

Cheers all, and thanks for all the help.
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Re: Bullet Aerodynamics

Postby pyrodude150 » Tue Oct 28, 2008 3:47 pm UTC

discarding-sabot rounds?
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Re: Bullet Aerodynamics

Postby Zamfir » Tue Oct 28, 2008 6:25 pm UTC

I seriously doubt you can improve existing bullet shape without massive amounts of testing. A good design would have both long range and high accuracy, and both depend on very complicated flow patterns with separated flow, something computers are not good at.

Of course, loads of tests have already been done, and there are probably loads of experimental/semi-theoretical curves out there giving the effect of bullet shapes on drag and accuracy at different Mach numbers and different wind conditions. But current bullet designers already know those curves, and it is not that likely you are going to improve signifcantly on them.

It seems a better idea that you use an existing bullet design that matches the characteristics of your rifle.

( I graduated on aerodynamic shape design, although not of bullets).
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Re: Bullet Aerodynamics

Postby wst » Tue Oct 28, 2008 10:02 pm UTC

From my minor bullet experience, tapering tails are the way to go. And a rounded nose, I think, but after a parabolic swoosh inwards. Or something.

I know more about firing the darn things than what they are :(
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Re: Bullet Aerodynamics

Postby Monty40xi » Wed Oct 29, 2008 2:14 pm UTC

wst wrote:From my minor bullet experience, tapering tails are the way to go. And a rounded nose, I think, but after a parabolic swoosh inwards. Or something.

I know more about firing the darn things than what they are :(

Wouldn't a tapered tail reduce how much energy the bullet can gain from the expanding gasses in the barrel?
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Re: Bullet Aerodynamics

Postby wst » Wed Oct 29, 2008 3:11 pm UTC

Monty40xi wrote:
wst wrote:From my minor bullet experience, tapering tails are the way to go. And a rounded nose, I think, but after a parabolic swoosh inwards. Or something.

I know more about firing the darn things than what they are :(

Wouldn't a tapered tail reduce how much energy the bullet can gain from the expanding gasses in the barrel?

It's negligible compared to the benefits from the lower drag.
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Re: Bullet Aerodynamics

Postby Monty40xi » Wed Oct 29, 2008 3:22 pm UTC

wst wrote:
Monty40xi wrote:
wst wrote:From my minor bullet experience, tapering tails are the way to go. And a rounded nose, I think, but after a parabolic swoosh inwards. Or something.

I know more about firing the darn things than what they are :(

Wouldn't a tapered tail reduce how much energy the bullet can gain from the expanding gasses in the barrel?

It's negligible compared to the benefits from the lower drag.

Hm, yes, there would be a tradeoff that depends on how far away the target is. Where is the break-even point, I wonder? 100 meters? 10? 0.1?
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Re: Bullet Aerodynamics

Postby TodayIsTomorrow » Wed Oct 29, 2008 3:29 pm UTC

What would the difference in drag be between a bullet with a flat tail and one with a full tail that tapers down to a point?

I've been working quite a bit on my machining software trying to come up with three different designs. One is the scaled design that I talked about earlier, with grooves around the body of the bullet cut as triangles with the angled surface in the direction of travel and the flat part towards the tail. The second is a spear like design that is simply symmetrical with the leading tip of the bullet the exact same shape as the trailing. The third is a hybrid of the two, with the symmetrical lead and trail, with the grooves on the body. Once I get back to the states, I'm thinking about have a batch of them made up and giving them a shot to see what kind of accuracy I can get out of them. A friend of mine has a .308, so I'll just modify the width of the bullet to spec. I've still got a few months, so yeah.

I'm really excited to see if this grooving works like I hope it will. The torpedo should work pretty well too, but I don't know how much stability I'll be able to get out of it, what with it being symmetrical, but I'm hopeful.

Thanks for the replies guys. If you've got any ideas you'd like to see tried, I'll get them machined and throw them down range happily.

Cheers
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Re: Bullet Aerodynamics

Postby wst » Wed Oct 29, 2008 6:55 pm UTC

Monty40xi wrote:Hm, yes, there would be a tradeoff that depends on how far away the target is. Where is the break-even point, I wonder? 100 meters? 10? 0.1?
Depends how fast the bullet is going. The preference is boat tails for extreme long-range, however - I've shot them at 600 yards and some of my friends shot them at 1000 yards. But the things I use at 25 yards are flat-tailed, and subsonic.

The nose shape of the bullet is actually not all to do with aerodynamics, thinking about it. The bullets I shoot in .22 competition has a flat nose, with a little bump at the very tip, in the centre. They make slightly smaller holes, and they are round holes, without tears radially. When the gap between lines on the target is smaller than the bullet, those tears are important stuffs!
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Re: Bullet Aerodynamics

Postby uncle_mcpeanut » Fri May 27, 2011 1:31 am UTC

alright if this thread is still up i doubt it i found some interesting things for your problem http://waterocket.explorer.free.fr/aerodynamics.htm
this sight shows a 45 i think. the shadow lines are actual Mach lines created by its movement and the anomaly trailing it shows how a tapered end would greatly improves a bullets aerodynamics
as for the rings on the bullet perhaps searching for a shadow graph of the .408 that the cheytec intervention uses
happy hunting :D
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Re: Bullet Aerodynamics

Postby p1t1o » Thu Jun 02, 2011 10:49 am UTC

My two pence:

The theoretical optimum shape for a supersonic body is well known, the sears-haack body.

Bullets tend to deviate slightly from this shape as a practical necessity for use in a firearm, for example, bullets need a cylindrical centre section to form a good seal in the barrel and to keep them aligned with the axis.

So I would have thought that to improve a bullet, you want to bring it closer to the optimum shape whilst still making it useable in a firearm.

I always wondered why bullets don't have more of a pointed tail though, I can't see how it would affect propulsion down the barrel.

PS: Those "ribs" that you mentioned, are you sure they aren't markings from barrel rifling? Adding texture or otherwise increasing surface area rarely decreases drag. Or perhaps they are there for another reason other than to reduce drag.
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