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barrel lengthwhat is the optimal barrel length for power and accuary , im not too sue if it matters but the projectile is is roughly 17 mm ,about the circumference of your average marble
Re: barrel length
How long is a piece of string? Optimal barrel length depends on projectile weight, valve efficiency, chamber volume, pressure etc. In general, a longer barrel gives higher velocity, which means shorter time to the target, less time for the projectile to be affected by environmental factors and gravity and flatter trajectory, reading to better accuracy. On the other hand, a longer barrel means longer "lock time"  a bigger time interval between you pulling the trigger and the projectile leaving the muzzle, giving the potential for inaccuracy as the launcher would have moved in that interval.
Re: barrel length
Measured vertically or horizontally?
We've gotten questions like this from other members, and the correct response is pretty much exactly what Jack wrote.
Though Technician1002 probably has something to add, but whether or not it'll be useful to you is another matter. Apparently he's actually found that in some cases a barrel too long is a hindrance, for reasons other than what Jack said.
As others have already said, many factors come into play when finding an optimum barrel length. In a pneumatic, there is only so much air in the chamber, and if that air cannot expand enough to fill the entire barrel, the projectile will actually be slowed as a vacuum forms behind it.
If you really want to find the optimum barrel length for your launcher play around with GGDT. It even has an optimization feature that calculates the optimum barrel length for you.
The optimum barrel length calculator I think just runs out the barrel length until the projectile stops accelerating, though there may be other factors that come into play. *cough*Technician1002*cough*
a 11 or 1.2 1 chamber/ barrel ratio and you'll be sweet.
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OK.. Link https://inteltrailblazerschallenge.wikispaces.com/Barrel+length+trim+method The best barrel is shorter than simply a CB ratio may indicate.
<sup>*</sup>for a given projectile weight and friction
More info on flow losses in a spud launcher is on this page..
http://www.crazybuilders.com/item.php?id=000002&type=project_section Look about 1/2 way down the page. Designing to minimize losses and maximize flow make the difference between an OK launcher and an outstanding one.
Power is the rate of doing work, F · V ,and the force here is simply the projectile base pressure multiplied by the barrel's cross sectional area (as the force and velocity vectors are parallel, the expression for power is simply the scalar product P = F*V ). Unfortunately, as projectile base pressure and velocity need to be found through the solution of what is, in the very simplest case, a system of three nonlinear partial differential equations in 4 variables, coming up with this power optimizing length is not a simple task. GGDT solves this system numerically, but is not open source, and thus is not adaptable to such tasks.
To optimize your launcher for power, simply find where the expression for power given above is highest, and cut the barrel off just after that. I am, however, left wondering what would possibly motivate you to want to do this, as power is an instantaneous quantity and says almost nothing about the muzzle energy or velocity the gun will achieve, or any other useful quantity. Perhaps you meant you wanted to optimize average power? Extending to the last section of your question becomes more difficult  optimal for power AND accuracy? Accuracy depends on barrel tolerances, mounting, aiming systems and projectile choice. Assuming all of those are done well, higher muzzle speed increases accuracy for the reasons mentioned by other posters, and the highest muzzle speeds for a chosen projectile result from optimizing the launcher's muzzle energy, not the launcher's power.
Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
As has been said, "it depends" is the best answer you can get in general. Just do some tests and/or use GGDT.
You can do some algebra with adiabatic process relationships to calculate approximate ideal CB ratios as a function of barrel friction and starting pressure, but to be honest, unless you know what you're doing and what assumptions are made, you really shouldn't use this. As for actually writing a simulation, for the low speed case (which most spud guns fit), solving ODEs is plenty adequate (see here). GGDT solves a system of ODEs, not PDEs, to the best of my knowledge. Solving systems of nonlinear PDEs is not trivial, especially when the boundaries are moving. The easiest approach seems to be the Lagrangian approach detailed in this book for the curious.
All spud gun related projects are currently on hold.
You know, that might help explain how spectacularly bad GGDT is for high speed flows. However, when I wrote my own toy program a while ago making the same assumptions you do, its performance diverged markedly from that of GGDT at the speeds where compressible effects became important, indicating that GGDT may not be quite as primitive as you suspect. If I were to guess, I'd say it's a quasi 1D PDE system solver which loses high speed accuracy due to its lack of boundary layers and shockwave formation on contact with breech parts and the barrel wall. There may also be some problems with the adaptive grid generation, or perhaps it takes a "best guess" approximation of the finite difference solution without one. "Solving" the system directly in the x,t space rather than transferring it into a proper computational space where the steps are constant could be inducing errors like the ones we see. I was going to contest your assumptions of negligible gas momentum and KE, but after some quick calculations it seems that most of the launchers built here are still low enough performance to get away with that assumption. Assuming constant pressure and temperature throughout the barrel, however, is downright wrong. Even at midrange pneumatic speeds, it just doesn't work like that. Compressible effects are becoming noticeable at even 100m/s, very slow for a modern pneumatic, and at between 200 and 300m/s simply cannot be ignored if reasonable accuracy is desired.
Spudfiles' resident expert on all things that sail through the air at improbable speeds, trailing an incandescent wake of ionized air, dissociated polymers and metal oxides.
I have no experience with pneumatic guns, but my idea is about 12 inches long. I have a valveless rocket that is almost 3 feet long, the air chamber its self is 1ft 5in, and it is 1 1/2 inches round. I use a 1/2 inch barrel and it is about 1 foot long, but I only use around 6 inches of it (because of the way I had to make the sealing bubble...). I am going to buy a coupling for it eventually to make the barrel much longer, big enough to fit a spliced 2 liter bottle. But to the point, my gun is fairly accurate with a diet coke 20oz bottle, and it can hit a target about 100 feet away at only 50 psi.
A shorter barrel works well with higher pressure and a small chamber. Due to the short distance breech to muzzle, the travel time is short. A fast valve is more important with a short barrel, than the long barreled cousins.
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