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6mm multishoot bolt action finished

Built a pneumatic cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Re: 6mm multishoot bolt action finished

Unread postAuthor: Ragnarok » Sun Dec 01, 2013 4:40 pm

bravootome wrote:actualy is not so hard to get supersonic, it's all about pressure.

Think about what the definition of the speed of sound is. To sound like Captain Obvious, it's the velocity at which sound travels in a material.
What is sound? The propagation of pressure through a material.
Hence, rephrased, the maximum speed at which pressure can propagate through a material is its speed of sound.

No pressure, no force, no acceleration. Hence, a gas's expansion is limited to its speed of sound. The only way to get supersonic with a pneumatic is for the launcher's internal speed of sound to somehow be higher than the external speed of sound.

Now, the speed of sound is proportional to the square root of gas pressure, but it's also inversely proportional to the square root of gas density - and the ratio of pressure to density for a given gas is defined solely by temperature. Hence, the starting pressure is of no relevance to the speed of sound.

So, with air, the internal speed of sound will only exceed the external one if the temperature of the gases is higher than ambient temperature. This is not generally the case, and the adiabatic expansion of gas actually results in a fall in temperature.

Hence, it's genuinely difficult to go supersonic with a pneumatic. There are ways it can happen, of which the most plausible is shock heating of the "dead air" between the valve and projectile allowing the temperature in the barrel (and thus speed of sound) to rise above that of the external environment.

But that's hard to get right. So, as a general rule, if you think you've gone supersonic with a pneumatic, you're probably mistaken.
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Does that thing kinda look like a big cat to you?
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Re: 6mm multishoot bolt action finished

Unread postAuthor: bravootome » Mon Dec 02, 2013 1:52 am

But that's hard to get right. So, as a general rule, if you think you've gone supersonic with a pneumatic, you're probably mistaken.[/quote]

I already enumerate the reasons i think i go least sonic ( maybe i exagerated with supersonicsonic) and i got many others but not allow to post due the rules.
But you say that the air should get out bypass the piston with sonic speed in order the bb to fly sonic(?) This is not true.
You developed a real theory to demonstrate i am wrong.
i do not get sonic all the time, i dont know why, cuz i made the piston to open pretty fast but i got a "thing" that i shoot at and i only penetrate it when the loud boom occure.
Unfortunately i cant get myself a chronnograph but iam working to make one so maybe next week ill finish it. i am 99% i shoot bb with sonic speed when it exits the barrel. And if i am 1% wrong then i will make it it shoot sonic speeds.
witch part of UK do you live ?
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Last edited by bravootome on Mon Dec 02, 2013 7:36 am, edited 1 time in total.
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Re: 6mm multishoot bolt action finished

Unread postAuthor: Ragnarok » Mon Dec 02, 2013 6:02 am

bravootome wrote:But you say that the air should get out bypass the piston with sonic speed in order my bb will fly sonic(?) This is not true.
You developed a real theory do demonstrate i am wrong.

The theory has not been "developed to prove you're wrong", it's an established principle - a gas cannot expand faster than its internal speed of sound (and has no way to push something faster than it can expand), so this is a limiting velocity in internal ballistics.

Were it so easy to exceed the internal speed of sound of a gas through Jeremy Clarkson's laws of physics (Which can be summarised as "An object with more POWER!!! has more speed"), the US navy probably wouldn't be quite so interested in building railguns (which have no such limits, as electromagnetic fields propagate at the speed of light) to improve their weapons range - but the thing is, even the heated gases produced by solid propellants have a limit imposed by their speed of sound.

I have no doubts you're capable of transonic velocities which can, potentially, produce a sonic crack through localised supersonic airflow over the projectile (see transonic and critical Mach number), but this a different matter from the projectile's actual velocity truly reaching or exceeding Mach 1.

I know reaching or breaking the sound barrier is an appealing target, but it's a lot more difficult to do with a pneumatic than just "more pressure" and more difficult to confirm than just "loud boom" - it's possible, but not likely; Hence the best thing to be is sceptical, particularly as a well built pneumatic like this doesn't have to break the sound barrier to be impressive.

witch part of UK do you live ?

Within an hour's travel of London, but I'd rather not be more specific than that.
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Re: 6mm multishoot bolt action finished

Unread postAuthor: bravootome » Thu Dec 26, 2013 1:49 am

bravootome wrote:6 mm multishoot qev finaly finished

I just shoot a new vid.....hope you like it....
Still few more things i want to add. AS i am keep shooting for over a month by now, i noticed a loose of accuracy( i was able to hit a dot by 10 m) and power. I was thinking my fiber carbon barrel got used and this is the reason for accuracy loos, but what about all that power ? so i unmounted the gun few times and increased the space behind the piston, still not big differance -not as powerful as it was when i made it. Till today when the weather got a little worm up to 5-6 dg. C. to my surprise the accuracy and power came back. So the air doesnt expand well at low temp. ... I'll pack it up for the spring time.....
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Last edited by bravootome on Thu Dec 26, 2013 8:06 am, edited 1 time in total.
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Re: 6mm multishoot bolt action finished

Unread postAuthor: jackssmirkingrevenge » Thu Dec 26, 2013 6:52 am



Great work!
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